First off, welcome to VSE!
Second, there is a section called HELP which is devoted to issues such as yours, which I'm sure a mod will be along soon to remind/inform you of. most likely, the thread will be moved over there when that happens.
In troubleshooting any synth, the first thing you want to do is narrow down the culprit or offending section of the unit. this rules out other "healthy" sections, and makes fixing the thing possible, i.e. no sound on an analog might be a bad VCA, so running another synth through a filter input would rule out either the filter or amplifier. Digital synths, however, are a whole different world as they 'mostly' rely on DSP and groups of IC's containing entire voices, as opposed to discrete circuits handling functions independently. In a repair philosophy this is called "Component Isolation".
Keeping this is in mind, I would recommend isolating your issue of the C notes not sounding to determine whether it's a bad voice, or simply a bad key contact on the keybed, etc. So try plugging in a KGB (Known Good Board) controller via MIDI in (another synth or controller) and see if you can get the C notes to play from the controller; if you're successful with this, then the problem (hopefully) has nothing to do with the sound engine section of the synth and lies somewhere between the electro-mechanical section of the keyboard and the means of sending note information to the sound engine, possibly a bad relay for that group of contacts or short in the wiring itself.
Another thing you can try is to transpose or detune the keyboard a semitone, so that C registers as a C sharp, essentially shifting the note assignment over or up one key, and see if you still have the same problem.
Lucky for you, this board is FILLED with DIY'ers and synthtechs who not only repair but also restore vintage analog and digital synths (Maschinegeist, HFinn and Rhino, to name a few) so you're in good hands. Gentlemen, you have the floor .
hope this helps, and good luck...