Hello acmail,
Thanks for reaching out to us here seeking help with iCloud mail.
Please start by checking if there is any maintenance or outages in your area: Apple System Status
Next, try using another device. This isolates issues between the account or device.
If you still have issue, please check the troubleshooting here: If iCloud Mail isn't working - Apple Support
If you still have issues, try on another wireless network, or turning off Settings > Wi-Fi, using only your cellular data. Please be aware this will use your cellular data until you turn Settings > Wi-Fi back on, but this step can isolate the wireless network out of the equation.
If you try another wireless network, make sure it meets these requirements: Recommended settings for Wi-Fi routers and access points - Apple Support
Cheers.