You can create an account on beaconcha.in to set up email notifications in case your validator loses balance (goes offline), or gets slashed.
Tip: If your validator loses balance for two epochs in a row, you may want to investigate. It's a strong signal that it may be offline.
1. Sign up at beaconcha.in/register
2. Type your validator's public key into the searchbar
3. Click on the bookmark icon
4. Tick the boxes and select Add To Watchlist