If I can assume that you've checked the physical connections, the next thing to consider is which operating system you're using. If it's Windows 98 (or earlier) your printer has to be switched on before the computer is booted up. If it's switched on afterwards, Windows won't accept that it's connected.
If those points don't help (and assuming that your printer worked previously) , then it's likely that a registry entry relating to your printer has been corrupted. Using System Restore should fix the problem. If you've never used it before, here's the route to find it:
Start > Programs > Accessories > System Tools > System Restore.
(That's the correct path for Windows Millenium. It might be a little different in XP. If you can't find it, just go to Start > Help and enter 'System Restore' for a link to the program). Then all you have to do is to select the most recent bold date when your printer was working and confirm that you want your system restored to this date.
In the unlikely event that System Restore doesn't work, try reinstalling the printer from scratch. If that doesn't work, it suggests that your printer may have gone to that great electronic paradise in the sky!
Chris