I’m trying to record a bass guitar on Artist 6.
When I set up a mono audio track there is no signal, even though my audio interface (M Audio fast track ultra) is showing a signal.
The strange thing is that if I set up a stereo audio track it shows a signal and records ok.
I’m not sure what is happening here as I haven’t had any problems in the past and haven’t changed any settings.
Any ideas what is going on?
make the correct bus- / track input settings as explained in the manual.
Lots to set up and bits easy to forget.
My routine is to:
1: Set the soundcard settings / Control Panel
2: Set the right driver in Devices setup VST Audio
3: In Cubase set up VST Connections
4: Check the inputs and outputs in the Inspector on the Project page.
5: Check the monitor button on the track or set the preference.
Now… what have I left out? 
I always do.
Conman gave you good advice, but I’ve found that even if I’ve checked all those areas, sometimes I’ve had to delete the mono input in VST Connections and then setup a new one. So, if Conman’s checklist didn’t solve the problem, try that.
I always set up a default configuration in VST connections and save that as a default (with a clear name). Most less expensive USB audio cards have a combined Mic/Line input, so I use the same inputs for a stereo channel and two separate mono inputs. In total you have three inputs - stereo, Mic/Instrument 1 and Mic/Instrument 2.
When starting a new project in Cubase I always make sure those defaults are loaded (and if not do it manually). This only takes a few seconds, but than you can be sure all inputs (and outputs and/or possible studio setup) is loaded.
It is easy to forget that when you use the default “empty” template the I/O setup in the VST connections are “forgotten” and you have to set them up again. To overcome this problem you can make your own default template with the correct setup.