Try going to help, then debug window, then type command "rewindchain 1000" and press enter, and it will rewind the chain back 1000 blocks, hopefully past the troubled block, and fix the issue.  This worked for me in the past on a similar issue.  I think it happens if you get on a forked chain on your client.  Also you can try deleting peers.dat as you may be connecting to a bad node.