Make sure your device/computer is connected properly to the internet by checking/ verifying that your router is on and connected to other devices.

If your email app offers the ability to synchronize/ sync data, verify if the app is configured/ set to sync automatically.

Logout and sign back into whichever email service you are using and try again.

If none of the above works, you can uninstall and reinstall the email app on your device, which should solve any temporary issue.

If the issue persists, reach out to the customer service or tech support team for the email service you are using.