Deleting the Digidesign database files usually solves this problem. These temporary files will be rebuilt automatically by Pro Tools after they've been deleted.

On Mac OSX:

1. QUIT ProTools so that is not running.
2. On the root level of every mounted hard drive, go into the Digidesign Databases folder and delete the "Volume.ddb" and "WaveCache" files.
3. Go to: MacHD > Library > Application Support > Digidesign > Databases > Volumes. Delete all of the files in this folder (they will have very odd names).
4. Empty the Trash and try using ProTools again.

On Windows XP:

1. EXIT ProTools so that it is not running.
2. On the root level of every mounted hard drive, go into the Digidesign Databases folder and delete the "Volume.ddb" and "WaveCache" files.
3. Go to: My Computer > C: > Program Files > Digidesign > Pro Tools > Databases > Volumes. Delete all of the files in this folder (they will have very odd names).
4. Empty the Recycle Bin and try using ProTools again.