Ok, so I dunno why it happened, but I have a bad fix for you:
Backup your wallet, dump your private keys (copy them down to a word processor or something), close nud, delete your wallet (OMG make sure you backed it up), restart nud, reload in your private keys one by one into a new wallet. Tada, fresh wallet.
There’s prolly a much better way to do this, but I’m pretty sure this will fix your problem.