Reboots again and again and again.

Hello,

It rebooted like 20 times and every time i start the Pc it reboots again and again and again.
When i start the laptop first there is the DELL screen Where u can press F12 for boot menu or F2 for setup after that the screen goes black with one "_" blinking for like a 2 sec and then the pc restarts.
If i tell my pc to boot from the CD it tells me "Press any key to boot from CD" if i press it boots from the cd if i dont press it start restartng itself again