Computer Won't Boot Up

A few ideas below should give you some reasons why your computer may not be booting up:

The hard drive is gone bad. If you are getting an error "No boot disk", it's possible that the hard drive is gone bad or there is a corruption in the OS.
Your power supply may not be supplying enough power for your PC to fully boot up.
Your computer can possibly be overheated, causing the computer to turn off every time it turns on.
If you cannot even get power from your computer, your motherboard could have gone bad or your power supply could be faulty.
If you are getting a blue screen of death (BSOD), try googling the error code (which is usually at the bottom starting with 0x). Hopefully you have another computer to google this error code on. If you don't, try taking components out of your computer one by one. If this BSOD is AFTER you past the Windows Logo, it's likely you have an infection on your PC. Try booting in Safe Mode.
A lot of times people experience this issue, and it's not always caused by a virus or bad hardware. I've seen in the past where people have simply ran updates to a program and this resulted in their computer being unbootable. I've also seen where Windows Updates have caused black error messages upon reboot, preventing the user from going any further than their manufacturer logo.

In the event you feel you have malware (trojan, virus, etc), you should always have your data backed up. Try booting into Safe Mode and seeing if you can get through there. Usually, if you're infected with malware, you won't run into the issues you normally run into while you're in Safe Mode, however, you are more limited to what you can do on the computer. Safe Mode is what I'd recommend running malware scans in. I say that because since malware has very low chances of running in Safe Mode, it'd be a very great idea to run a full scan and detect any and all infections.

Majority of the time, you can boot using a boot CD to get into your PC if you can't get to it through your operating system (OS). Boot CDs normally give you much more troubleshooting features than you would have just booting into Safe Mode or any other modes in your OS. Boot CDs allow you to boot to another OS (stored on the CD) and gives you the capability of browsing your internal hard drive. This is a very common procedure I perform when I run into clients who have viruses, can't boot their PC, and they need files off of their hard drive. I'd normally boot into the Boot CD and just extract their data from the internal hard drive and drag it to my backup external hard drive. Keep in mind that whenever.