Hello,
I connected yamaha moxf 8 with cubase with usual setup. I wanted to record an audio track in cubase by using moxf8 sounds. I set Stereo In 2 bus setup and all. Whenever i click play or record button in cubase, a random key of a voice produces sound on moxf.. if it is a sustained voice like violin, it sounds continously till i click stop button. problem occurs from today. Till yesterday, everything worked well. I didn't customize any option on moxf or cubase. Please help to solve this issue. Thanks.

This is always caused by a MIDI Port configuration error.
The MOXF when connected via USB to a host computer, communicates on 5, count them, 5 MIDI Ports.

A MIDI USB Port is 16 Channels IN, and 16 Channels OUT... the equivalent of two 5-pin MIDI cables... so think of your USB connection between the MOXF and the host computer as replacing 10 standard MIDI cables (as well as 4 outs + 2 ins of standard audio cable connections)... all traveling digitally through that one USB connector!

MOXF MIDI Ports
__PORT 1 - is for the internal MOXF synth engine. It should be the one used for musical performing: Note-Ons, Controllers, Tempo...
__PORT 2 - is for using the DAW Remote Control Surface function. This will re-task the front panel buttons, sliders, knobs and keys to send MIDI commands that can be used to operate DAW software like, Cubase, Nuendo, Logic Pro, Sonar, Digital Performer... etc.
__PORT 3 - when the MOXF is set to MIDI I/O = USB, rather than leave the 5-pin MIDI Ports idle, Yamaha made them into a MIDI Interface for an external device. Data simply ‘passes through’ the MOXF going to and returning from the host computer. Anything plugged to the 5-pin MIDI IN will arrive in your DAW as “MOXF6/MOXF8-3 (Port3)”; anything connected to the 5-pin MIDI OUT will receive data from the DAW as “MOXF6/MOXF8-3 (Port3)”. Say you wanted to use a different keybed to play your MOXF sounds... set Port 3 as the Track’s MIDI IN, and set Port 1 as the Track’s MIDI OUT. With LOCAL CONTROL OFF, you can see how you can choose either keybed to play whatever destination synth.
__PORT 4 - is used to run the graphic user interface of the Yamaha MOXF6/MOXF8 Edtior VST/Stand-alone” when setting up the MOXF VST, this Port 4 is used so that hardware/software synchronization is 1:1 with the MOXF hardware
__PORT 5 - is used exclusively for the “Yamaha MOXF6/MOXF8 Remote Editor” which allows you to create controller templates for VSTi software synthesizers that you may run in your DAW. It comes with some 50 Templates, but you can easily create your own templates.

In your case, you have (inadvertently) made PORT 2 active (the one that turns the front panel into a Remote Control Surface) in the wrong place. The MIDI commands sent by the front panel of the MOXF are re-tasked on Port 2, so hitting a front panel button sends a messages on Port 2. When properly configured this discreet message allows the the MOXF front panel to act as a remote control for the selected DAW software. The [DAW REMOTE] function needs to be active.

A button makes a perfect On/Off switch, so a Note-On message sent on Port 2, if improperly routed and combined with Port 1 data as an INPUT to your MIDI Track, will turn front panel buttons into Note-Ons, your Keys may start to Open DAW windows, or reassign function (seemingly random chaos — but it is not random, it is very specific note-on events... which should be the clue, it just misdirected commands). However, that same button’s message when interpreted by the Remote Control template, might turn On/Off some DAW option. Each DAW manufacturer has their own unique configuration mapping of functions.

Where/how does this happen?
It happens when you select “ALL MIDI INPUTS” as the source for your MIDI Track. If you are going to use more than one controller Cubase lets you define which of your controller’s Ports has MIDI INPUT to your MIDI Tracks and, importantly, which ones don’t.

You get to define exactly what “All MIDI Inputs” looks like.

How can you fix it?
Unfortunately, you did not tell us which version of Cubase you are using...
But if it is a version of Cubase 8 or earlier - go to DEVICES > DEVICE SETUP... > MIDI > MIDI PORT SETUP... >
If it is a version of Cubase 9 or later - go to STUDIO > STUDIO SETUP... > MIDI > MIDI PORT SETUP... >

once there, find the column labeled “In ‘All MIDI Inputs’”
Here you want to mark ONLY those Ports that you want as MIDI inputs to your MIDI Tracks.
For example, if you wish to use the MOXF keyboard you would mark “MOXF6/MOXF8-1 (port1)”
If you had a second controller connected to the MOXF’s 5-pin MIDI IN, that you would like to use, mark “MOXF6/MOXF8-3 (port3)”
If you have another USB Control device connected to the computer as a controller mark it’s Port, as well.

But ONLY those PORTs you want to record MIDI information from should be marked (defined) in this “In ‘All MIDI Inputs’” column.

You obviously have Port 2 marked, unmark it.
Unmark Port 4
Unmark Port 5
Only mark Port 3 if you have a second controller connected to the 5-pin MIDI IN on the back panel of the MOXF.

If you want to use the MOXF as a Remote Control Surface... look in the left column of the SETUP screen for the Folder labeled “Remote Devices” — it is inside this folder where you tell Cubase you want to communicate to the REMOTE CONTROL layer on MOXF6/MOXF8-2 (Port2)” for both MIDI In and MIDI Out.

Hope that helps...although you may not have changed anything, you failed to check your MIDI routing— hope that helps you understand the MIDI Port system of your MOXF and how Cubase helps you define what goes where.