Hey dm218,
Welcome to Apple Support Communities.
Are you experiencing an issue with the iCloud Mail account missing, or it is that emails are missing when you access the Mail app?
If you're not seeing the iCloud Mail when opening the Mail app, make sure that iCloud Mail is still enabled in Settings > [your name] > iCloud Mail. If it is on, toggle it off for about 15 seconds and then toggle back on. Toggling it off and back on refreshes the iCloud Mail account.
If you still experience issues with iCloud Mail and you haven't done so already, restart the iPhone. Restarting refreshes the iPhone software and has been known to resolve software related issues. To restart, follow the steps below or here: Restart your iPhone.
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
If the steps above didn't help, follow the steps here: If iCloud Mail isn't working. It provides great troubleshooting steps regarding iCloud Mail and may help resolve your issue.
If you're experiencing a different issue, providing more details may help you get more answers.
Take care.