It's usually a driver issue (the drivers can randomly get futzed up, and the iPhone, iTouch and iPad all have different drivers, so one can work and the others not)
Unfortunately there's no single fix, rather several things that can work and you just have to find the right one.
If you're using a hub or USB extender stop and plug it directly into your computer.
Try going into the control panel and 'add/remove programs', and remove the 'apple mobile device' driver if you see it. Try plugging it back in and see if that works. If not, try uninstalling and reinstalling iTunes.
if that doesn't work look for articles on the apple mobile device driver and see what else they suggest.