What has possibly happened to my dad's email? (details inside)

My dad recently tried to connect to the family’s smart TV with his email address. This is an email that he has for a long time and he isn’t tech-savvy enough to make any significant changes to it. Today he noticed that his email has slightly changed: let’s say his email goes something like familyemail1234567@gmail.com, and now what he is seeing is familyemail1880@gmail.com. But his name on the email remains unchanged. This name change can be seen on other places like his Gmail app too. He insisted that he did nothing to change the name. He can no longer use his password. When we checked the password it said that he had changed it 7 months ago. This was too long ago before he connected to the TV.

He is very worried because his email is linked to important things like his bank, so in the worst case scenario his email has been hacked and someone could have control on a lot of things. Has the email been hacked? What can we do in this situation?