Check if the [USB connection] is set to [PC Remote] in the setup menu of the camera.
If the camera is set to [PC Remote] and is also recognized as removable disk when set to [Mass Storage mode] it is possible that a required Windows service is not properly started.
Check if the service [Shell Hardware Detection] has been started.
If it is not started the camera will not be detected. Usually the service is started by the Remote Camera Control software automatically, even if it has been stopped or paused before.
How to check the Shell Hardware Detection service:
Windows Vista / Windows 7:
Right click on [My Computer] -> [Control Panel] -> Services
Select [Control Panel] -> [System and Security] -> [Administrative Tools] -> [Services]
Click on the [Shell Hardware Detection] and check if the service is running. If not you can start in manually.