My Mac mini 2012 with lion os x won't start up and stays on the grey screen with the wheel loading and sometimes while on the grey wheel screen it just instantly goes to no signal with the computer on the monitor and if I want to try and start up the computer again I have to hold the power utton for 5 seconds and then press it again. The only thing I can access on the computer is my window 7s which I have on bootcamp which I access by pressing alt on start up and this seems to be working fine but I want to be using my apple operating system. This just happened randomly one day and I hadn't used windows 7 through bootcamp before this happened for months but now I'm forced to use it. Please Help Me