It is inevitable. Your computer will freeze or BSOD (blue screen of death) some day.  Hard drives and other components will fail. Knowing what you can and cannot do will save time and money. Let's get started. BACK UP YOUR COMPUTER!  It might be too late if you are reading this but hopefully not.  Get an external hard drive or an enclosure/internal  hard drive and backup monthly at the least.  Backup software can be free as well, try syncback for Fbackup4. Now that you are backed up, this isn't the end of the world.  If you didn't back up, it's still not the end of the world.

1. Is the computer making any grinding or clicking noises? If so shut it down and don't turn it back on. Most likely your hard drive is toast. Sorry, it was only a matter of time. Time to get  a new hard disc and restore your data. You can recover some data from the non-bootable drive in some cases as well as have a 3rd party company do this (expensive) but that is another article.
2. If the computer has frozen at random while running some program while in your operating system (windows) write down the error if there is anything to write down and shut down and then start your computer.  Notice I didn't say reboot.  Most of the time this will fix it. If not move on to the next step
3. If your computer is hanging up during boot up try booting into safe mode by tapping the F8 key every few seconds while the computer is starting.  If you can boot to safe mode go to step 6
4. If you can't boot into safe mode try resetting the BIOS to it's default settings. This can be tricky because they key to do this varies but usually it will display it for you immediately during start up. Esc, delete, insert F2 and F12 are common keys for getting into the BIOS. Once in the bios, navigate around and find an option similar to "reset BIOS settings to default"  Flashing the BIOS might also prove helpful if you are familiar.
5. Try booting from a recovery device such as a Flash drive, operating system disc, or recovery CD and follow the onscreen instructions. Google Hirens Boot CD or UBCD
6. Once in safe mode or back in your operating system run scan disck, virus scan and malware scan and check your logs by going to Start -> control panel -> Administrative tools  -> Event viewer.  See what those things turn up by simply Googling them and reading about any possible solutions others can recommend.

There are many things that can go wrong with your PC. If it's software related, your chances of recovery are good given some time and patience. If it's hardware and you're hearing noises coming from within the computer your chances are quite a bit less.  Follow the above general guidelines for restoring your computer and use google to find answers about specific issues as they come up. Good luck.