First you need to make sure your wallet is working correctly, read Checking Your Core Wallet is Working to find out how.
Assuming you’ve followed the guide linked above, in the wallet toolbar, click File -> Repair Wallet
When prompted click “Yes” to restart and repair the wallet.
Once your wallet has closed you’ll need to reopen it. After opening it you will see a message confirming that your wallet has been repaired.
If your wallet balance is still incorrect you should reach out for support on our Discord server or the /r/Navcoin subreddit