You probably need to reinstall the driver for the TV tuner. So the first thing you need to do is to find it listed in Device Manager (unless you've got an installation disc for the TV tuner, in which case you can just reinstall the software from there).
Click on Start.
Right-click on Computer.
Click on Properties.
Click on Device Manager.
(Look for any yellow exclamation marks, which might suggest a driver problem, but don't assume that everything's OK if there aren't any). Double-click on the relevant section (which is likely to be Sound, Video and Game Controllers).
You're seeking an entry roughly like this one:
http://www.sevenforums.com/attachments/media-center/133695d1379292262t-no-tv-tuner-detected-tvtuner.jpg
Once you've (hopefully!) found it, right-click on it and click on Update Driver Software.
Once that's been done, restart your computer and try again.
(If you find the TV tuner listed but the 'update' option doesn't work, you'll need to find the driver software on the manufacturer's website - ask for help if you need it, telling us the make & model. If there's no tuner listed at all it might be a physical connection problem, or simply a dead tuner circuit, which could be a bit tricky to fix!)