First up, look to use a non ISP provided email. It just locks you into their service and in general the security options provided are fairly minimal. Get a google account with 2FA perhaps?
For your issue first change the password to your account (use some symbols and a bunch of letters). Generally if your account has been 'hacked' you'll find the hackers first action is to change the password so you can't login your self.
If you can login and change the password, then you haven't been hacked.
well, this has just happened to me..
50 plus emails returned to me, saying they cannot be delivered...
I logged into webmail and still had access.. try as I might to change my password, there is no link to do it! Now, I have lost access to my email account...
On hold with an optus tech, via chat, to get help, but he has gone away for 5 minutes.. not sure what to do now???