Hi,
A blue screen can result from a lot of problems including hardware failure or malware.
What is your Windows version, what service pack ? Have you recently installed a new hardware device ?
If you have the time, you can copy the error message the blue screen displays and look up on a search engine, it usually provides good pointers...
(to stop automatic reboot after a blue screen, you can press F8 at the Windows boot)
Your question is too wide as is.