Pc turns on and off

Hello, I am new to this site so hopefully i'm posting in the right place. Anyway my computer (I did not build it myself) which has been working fine since I had it, and had no issues yesterday only recently started having the fan speed up a bit for about 1 or 2 minutes upon start up and now...

So today I built my first gaming pc. I didn’t encounter any problems but when I turned it on for the first time, everything seemed to work fine I.e case fans and cpu fan spinning like normal but after about 10 seconds it turned off for about 3 seconds then came back on. The cycle the. continues...