my desktop Mac OS X is not starting, I get the gray screen with the apple logo and a bar below and then shot off by itself.

My Imac is not starting , I get a gray screen with the logo and a bar below and about one minute the bar desapier and shot off by itself. I try evething that I can think of and not luck. I did disconect all accesories and still the same, I use the start up disc and same.. now I can not get the disc out.

Please advise what I need to do to solve the problem. Recently this computer was upgraded from 10.4 to Mountain lion and add more memory.