IvanF wrote:
Could it be that your drive has been remapped to a different letter? It can happen if you have added another drive or used a flash drive or something. So if Lightroom is looking for a folder on F:\ for example and now it sows up as G:\ in LR's Library panel right click on the top folder that has the ? and a "find missing folder" dialog will pop up. Navigate to the folder on the hard drive and LR will do the rest.
Hope this helps. :)
I was about to suggest the exact same thing, because I've had the EXACT same thing happen to me. I opened Lightroom one day, and suddenly, all my photos were gone. Turns out my system decided to give my external HD a new drive letter... I suspect this might be the problem...