Mac pro problems starting

I have had this computer 8 days. Mac Pro 15 inch retina display- the works. The computer wouldn't turn on so I figured out how to do the control command PR and it works now. My question is - what is the problem. Should I be concerned? It there something I need to do to make sure I don't have a lemon since I just got it!? I ran the apple diagnostics and everything said its fine. What gives?