Keep calm, you didn't say which wallet you're using, is it Bitcoin Core? Searching on google, other people had this same problem. Their solution should work for you too. First of all I recommend that you back up your wallet.dat to a safe location so that you do not risk corrupting the file.
The solution outlined
here solved the user problem, maybe it works for you too.
Try starting Bitcoin Core with the -reindex option. If that doesn't work, go to the data directory and delete the chainstate and databases folders.
