Based on which processor you have, you can buy a new one from many online retailers, it will come with instructions on how to install it. Do not run your computer till you get it as it will probably overheat. Put the new one in, using the same power plug as the original.
If the computer is within warranty then ask that it be repaired/replaced. Otherwise, a new fan can be purchased and fitted - usually just a case of clipping into place.
If you know a lot about PCs, replace the fan. Otherwise, take your PC to a repair shop and ask them to replace the fan. Replacement fans typically only cost $20 or so, but with a broken fan your entire computer could be ruined. Don't use the computer until the fan is fixed.
If you don't know someone savvy in computer repair look in your local yellow pages for "Computer Repair". Make some calls and price comparison and then have your cpu fan fixed and/or replaced. This is something that can be taken care of easily and should not be expensive.
Try blowing air on it first, as it sometimes gets stuck from heavy dust or a little piece of paper, etc. If that fails to start it going again, you'll need to replace it so the computer doesn't overheat, or call a technician to replace it for you.