If for whatever reason the following doesn't fix your issue, join the Discord server and ask in the public channels only for help. Do not respond to any DMs as scammers might try to trick you into giving them your funds!
If you happen to be on the wrong chain (typically due to not updating your wallet in time before a mandatory update), your wallet may show an incorrect balance and transactions may fail to send properly.
How to Resync
First, verify that you are indeed on the wrong chain by comparing your block height to the explorer. If they do not match, take the following steps:
Make sure your wallet is not running and navigate to the XDN data directory.
If you don't have a backup of your wallet.dat file, now would be a good time to make a copy just in case you accidentally delete it while doing the next step. Check the Wallet Backup page for more details.
Delete the following from the data directory: - blocks (entire directory) - chainstate (entire directory) - banlist.dat - peers.dat
Start the wallet and let it sync. Using a broadband internet connection, it should take about 15 mins to complete.