freezing and then came black screen

recently the monitor freezes started, then i had a bad memory stick , now every few minutes I get the black screen and the only thing to do is shut off with the main power button. what should I check and where to start, thanks.

This can be caused by a number of things.
The first thing to do is to find out if it is hardware or software related.
The best way to do this is to boot into the bios and leave it there for a bit.
If the problem arises when let in the bios screen then you know it is hardware.