I have a project open containing Instrument and Audio tracks.
I’m getting an occasional click (not consistent) with the audio, and then…the audio tracks go SILENT!!!
They are not muted, there is nothing soloed. The wave forms are still visible. I can make all kinds of routing changes, and to no avail.
The instrument tracks are unaffected!
The only solution that I have found is to shut down and reopen Cubase.
Could try listing your full spec mate… it might help just a little 
mac/pc? 32/64Bit? etc…
Sometimes a corrupted file can temporaily seize things up. It could be a faulty audio drive, maybe a physical connection, I even had a problem with a DVD writer once that was trying to take over the system which caused some dropouts
1 Try making a copy of the project on another drive, do you still get these dropouts?
2 Test your drives are ok - I use a free utility (on PC) called HDTune to check all drives periodically - I discovered a dodgy S-ATA lead this way
3 Use Backup which makes fresh copies of all audio in use and try working from the new project (on the same drive) does this solve it?
4 Question - does it affect only one project or many?
5 Question - do the dropouts occur at the same place in project timeline each time
6 Question - what do your CPU and disc meters do at the point of dropout
As Mat Jones says, specs will help, are you running very low latency?
Do definitely make a safe backup of everything first!
P