Happened to me before. Thsi works (taken from a different forum): "Fortunately, the fix is easy. Exit DW, delete the WinFileCache*.dat file in the Configuration folder, and then restart DW.
When tracking down this problem I found posts from other people who recommended deleting the entire Configuration folder. This works but you end up deleting more files than you really need to.
The WinFileCache*.dat file will be recreated next time you start DW 8.
NOTE: The "Application Data" folder is hidden by default, so you'll need to change your Windows Explorer Folder Options to View Hidden Folders.
Always remember to back up your Dreamweaver 8 folder before deleting files!"