My email won't let me log in on my phone but is fine on my desktop.

All of a sudden, when I try to log into my email account on my phone, which is something I've done using a browser for literal years - like a decade - it tells me my password is incorrect. Never mind that it's saved in my iphone and hasn't changed. When I log into my Xfinity account on my desktop using the existing password, it lets me in, no problem. This is only a problem on my phone. I've tried using Firefox and Safari, as well as the app. Help?