Does it give a series of beeps before locking up? If so you can search the internet (or ask here) for an explanation of what your specific number of beeps mean (you'll need to know your BIOS manufacturer too).
It sounds like it could be a problem with your Memory. Have you added/removed any sticks of RAM recently? If you have more than 1 stick in your system then you could try to remove all but 1 of the sticks, making sure it's sitting in the right slot (if your motherboard is picky) and seeing if the System boots differently. Try the other sticks one by one to see the results. If the System boots, start adding the remaining sticks one by one to nail down the offending one.
There could be any number of other things going on, but dying/dead Memory sticks are fairly common.
