You can create an account on beaconcha.in to set up email notifications in case your validator loses balance (goes offline), or gets slashed.
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 search bar
3. Click on the bookmark icon
4. Tick the boxes and select Add To Watchlist