Steps to follow to correct wrong balance displayed on the wallet:
Check your address on block explorer (type /explorer on chat for the link) to make sure your coins are still there. If yes, continue with the next steps.
Open wallet configuration file from Tools menu (fusion.conf), add the line "zapwallettxes=1" (without "s) and save the file. Use notepad (or your standard text editor software) to open the file if asked.
Restart your wallet. Wait while it rescans the blockchain. It takes a few minutes.
When complete, you should see your correct balance on the wallet.
You might want to remove the line from fusion.conf once your balance is back to normal, so that it doesn't rescan the blockchain (takes a while) each time you restart your wallet.
If the problem persists, please don't hesitate to ask for help on Telegram!