Make sure that you are connected to the internet.
Make sure that your email account is set up properly.
Can you send email from your iPhone?
If that works properly I would try
Removing the account from your watch
Then force restarting your iPhone by whatever method your version calls for. As soon as you see the Apple logo force restart your watch by holding in the crown and side button for 10 seconds till you see the Apple logo then release. Log into your Phone and wait for your watch to boot, then log into it.
Test the some app on your watch that needs and internet connection to work if it does then
Test your email account on your iPhone again, if it works then add the account back to your watch and test