This is usually caused by corruption in the session document or a corrupted database.

If you have created any catalogs you should back them up. Go to
Program Files/Digidesign/Pro Tools/Databases and copy the catalogs
folder to a new location. you can put it back after doing the
following. (Only if you have created them will you see a Catalogs folder)

- Trash the DAE prefs folder: Located in the C:/ Program Files / Common
Files / Digidesign / DAE

-Trash all files located in the C:\Documents and Settings\your user
name\Application Data\Digidesign

-Trash the folder called Digidesign Databases on the root of all
mounted volumes.

-Trash the Databases folder located at Program Files/Digidesign/Pro Tools.

If this does not resolve the issue create a new session with the
same start time and sample rate. Use import session data from under
the file menu to bring the tracks from the problem session into the
new session. If importing all the tracks at once does not work try
doing them in small groups to see if you can identify the problem
track/s. Choose to try copy the audio from source media to make new
audio files.