Hiya:
If that error continues after re starting everything go to: %appdata% and in the [navajacoin] folder back up the wallet.dat (always do that!!! that's where your coins are!!!)
Then delete that folder AFTER YOU BACKED UP YOUR WALLET.DAT folder.
Then re-start the wallet and place the backed up wallet.dat in that folder. Will ask you if you want to replace the file, say yes.
It will take ages to download the blockchain again, I suggest you to download the whole blockchain from website and replacing it.

REMEMBER TO BACK UP THE WALLET.DAT file

Salud!