The screen stays black on MacPro tower on startup
My MacPro tower does not light up the screen after startup. The chime goes off, but nothing happens! I have tried holding the P and R and COMMAND keys down on startup, but that made no difference! It has Yosemite10.10 installed, but if I can get it going I want to install SNOW LEOPARD on it as Yosemite seems to have too many bugs in it yet.
Anybody has any good advice for me?