Which camera make and model?
See if yours is listed
Which exact version of Lightroom are you running? Current version is 4.3.
Hi, Camera:- Canon 500D & LR4.3 The camera is listed as compatible
Should work OK then.
Have you tried rebooting the computer just to see if it makes any difference?
Which operating system? I assume Windows 7 or Windows 8?
But using Lightroom to tether will NOT allow you access to the camera controls, nor can you see the live image, which is what I think you thought you had. You can SEE various camera settings in the tether panel, but you can't change any of them using the panel. Perhaps when you first tested it you were actually using Canon's EOS Utility, which DOES give you both a live view picture as well as the ability to control the camera settings?
I have the same problem. It came with Windows 8. I think that this is a Windows 8 related problem as EOS Utility don't see the camera either. But the memory card of the camera is accessible by the Windows Explorer. In Windows 7 both Lightroom and EOS Utility are able connect to the camera. I tried the camera with and without the memory card. And booting in between.
Fortunately tethered shooting works with my laptop in Windows 7.
** edit **
I got it working. Earlier I used both RAW&JPG mode. I replaced it to RAW only and set the camera to live view shooting. After restarting the PC, LR found my camera and got the pictures. And without the memory card. All settings must be set in the camera, LR can be used for shutter only. Of course the camera can be used for everything.