Several reasons:
a) You have given no details of what is happening. We don't even know your OS. Have you got any error messages?
See here which details are of interest / necessary: http://forums.adobe.com/thread/1042146
And what does "completely vanished" mean? Where have you looked?
b) Also, please be aware that this is a user-to -user forum for Lightroom. What we know we learned from using Lr. We are not Adobe. The Cloud is an Adobe thing, and we don't know much about it.
If you are on Windows try this: Go >Start >All Programs and look if you can see the Program <Adobe Photoshop Lightroom 4.3. Click on it to open it.