Q: Brand new Mac Pro is completely blocked

Hello everyone. I've buyed yesterday an MacBook Pro (middle 2012). It is my first Mac, but I have already repented. Everybody told me, the Mac is another world. For the first evening they were right. The Mac was that fast, I couldn't believe. But today the story changed. In the afternoon I've seen for the first time the colored pinwheel. Then, when I was on the internet, another time. Until, while I was working with final cut, the reason of why I've buoyed a Mac, a kernel panic. When I tried to switch my Mac on it stayed for 10 minutes on a grey screen page. I've turned it another time off, but the same problem. I've tried with the safe boot. Nothing. When I finally reset the pram after a while the Mac turned on. I went on Mozilla to search a solution or an test, but my Mac got stacked. I've tried everything, but the  onliest solution was to pull the "switch off" button. Of course it didn't went on (it was blocked on the grey screen). When I tried to reset the pram a question mark appeared. So I decided to make an hardware test, but the result is that the Mac doesn't have problems. Now, I don't have any apples cd's or other stuff. Of course I have still a lot of memory in my 500gbhard disk and I have 4 gb of ram. What should I do?

P.s. Go to the apple store is the last thing I will do

P.p.s it is a second hand Mac, but I've sen it has been used only 7 hours.