You can try a bit of Troubleshooting, and noteing any changes that has happened intentionaly/unintentionaly in a before and after situation, then posting it here.
If you didnt get what i said, then just read Howard's post
Software problems usualy have symptoms of being able to use the machine to some extent before failing, and usualy triggered by a bad code in the program. Sometimes it will just bring you back to where you've started from and sometimes it just packs its bags and leave or not do anything at all. Usual solution is to repair or reinstall that peice of software.
Hardware is in addition to symptoms in software i metioned above (since hardware needs "drivers" which is in fact specific software to run the hardware) and that its sometimes easier to notice (assuming you use Windows, a yellow ?, !, triangle or red X in device manager etc) and off course being able to physicaly inspect it for burns, damage, inconsistancies etc. Booting another self-sufficiant OS like Knoppix can help eliminate the Driver problem being the culprit in a hardware situation, since it just uses generic type drivers, then you'll know its the OS you have on that has potentialy the wrong drivers etc. Depending on the hardware involved it can be as simple as swapping the part(s) out or if its not cost effective getting new parts/pc..