You can start USDe.exe with the argument -rescan

On Windows you can do this via

Winkey+r > C:\path\to\usde.exe -rescan > [Enter/OK]
  OR
Create a shortcut of usde.exe > Right-click on shortcut > Properties > in Target: append " -rescan" without quotes

(Tip: rename the shortcut to "Fix USDe sync issues" or something like that, and you can click this icon if it ever happens again)
 
If that doesn't work, just delete the blockchain files and redownload.
Close USDe.exe
Go to your data directory and delete the database folder, blkindex.dat and blk0001.dat files.  
If you still have issues, you should delete peers.dat as well
By default you can get there by doing: Winkey+r > %appdata%\usde > [Enter/OK]

(Tip: your wallet.dat file is also here.  Be sure NOT to delete it!  Back it up!)