You need to remember how Cubase works. If you create MIDI track, and record from your master keyboard, and the data appears in the track, in the inspector you can, depending one what gear you have connected and set up, send it out of a MIDI output, or if you have started up a VSTi - either as an instrument track, or as a VSTi in a virtual rack you can send that MIDI track to those outputs too. If you create an instrument track, you can take the track and drop it onto the instrument track. The data is the same. The instrument track can do a few things a MIDI track can't, but the differences aren't really important for this problem here.
In the inspector - check to see what destinations Cubase is able to send that MIDI track or Instrument track to. You might see a list of running VSTis and real MIDI outputs - I suspect you've just lost the routing - once you route the MIDI data to an output, it will appear in the mixer, with an associated fader. Best guess is that you are not connected to a destination!