Doesn't the Camera app open or close by itself? Or do you get an error like “Can't connect the camera”? One of the first things you can do is reset the camera settings to rule out any conflicts.
To do this, go to Settings >> Applications >> Manage applications >> Camera. Click on Storage and select Delete data,
If you want to rule out that it is not a problem with the camera sensors, then install a third-party Camera application, or use the same CTI menu (as we saw in the first item) that will allow you to check the status of each of the sensors .