Common reasons for a computer not booting include a power issue, memory module seating problems, loose cables and a problem with the operating system. Boot issues can be caused by both hardware and software issues, or a combination of the two.
Check to make sure that the computer is plugged in, and watch the motherboard lights when the computer is powered on. If these lights do not power on, the power supply may be broken, or there may be a problem with the motherboard. Check to make sure that the RAM modules are seated properly and that all cables connecting the various parts of the computer are fully seated. If there are no apparent hardware issues, and the computer starts, but the operating system fails to boot, the issue most likely stems from a software issue.