start up problems

When I start up, I press the on-off and the computer starts after about 1 or 2 seconds. The apple appears and the progress bar starts. After about 30 seconds, screen goes black and computer has shut down. I have to wait a few minutes and then press on-off again and it starts normally through to really to operate. Did not do this before and has been doing it for about two weeks. Could it be the Yosemite? Any help? Thanks!