Be sure you are working with the latest wallet edition.  If you are and you still have the problem, close your wallet, and consider this:
You could delete and redownload the entire blockchain fresh. You do this by deleting all but wallet.dat from the dat files in the %appdata%\PotCoin folder. DO NOT DELETE WALLET.DAT, BUT MAKE A BACKUP OF IT WHILE YOU'RE IN THE FOLDER, AND KEEP IT  SAFE SOMEWHERE.  You enter the %appdata%\PotCoin in the start-up search box where you enter run commands.  Open subfolders to remove dat files there as well.  When you are done deleting these dat files from %appdata%\PotCoin close all these folders as well.  Then when you go to restart your wallet it will say something like "the blockchain is corrupt would you like to redownload it". Say yes.  The blockchain will start redownloading from day one.  This could take a long time, but usually solves your problem.