I've found that this happens when Outlook Express wants to do its stupid "compacting folders" thing.
If you are lucky then the dbx files are still intact.
Do a search on your PC for *.dbx, then cut and paste the dbx files to another location.
Open Outlook Express and create folders with the same names as the dbx files that you have moved.
Close Outlook Express.
Copy and paste the dbx files that you moved back to the original location overwriting the ones that will have been created by making new folders.
Re open Outlook Express and you should have the contents back unless the files were corrupted.