Had it happen before during an update. Make sure everything closed down/restarts properly, go to
your NAV download folder and open wallet from there. I had to try 2 or 3 times a few months back, but
it came back up and working fine now. Make sure you have the most recent nodes for the 3.9 wallet and
check your CONF file to verify everything is sequenced correctly with latest nodes.

https://chainz.cryptoid.info/nav/#!network