If you go into Device Manager you should see it listed under Imaging Devices, (for XP anyway).
If it's not there and there is no Unknown Device listed in Device Manager then it could be that when the monitor was fixed, they forgot to plug the cable from the webcam back into the motherboard or back into the webcam.
In which case, if it's no longer under warranty you can undo the case and plug/check the connector back into the motherboard or webcam as needs be.
If it's still under warranty, take it back to whoever did the repair and ask them to check it because the repair should be under at least a 30 day warranty.
If it's listed in the Device Manager and not shown as Disabled then download the appropriate driver again, (there were two types of webcam used, get the right driver), and reinstall it to see if that makes a difference.