This is potentially a very serious issue, and could lead to identity theft.

 

It goes beyond just affecting the email, or sites where that password may have been used.

 

The "hacker" has access to the password, so they can also check your mail, and probably has already downloaded all mail in your inbox.

 

They could then visit various banking, e-commerce, etc. sites, likely using a robot to request password resets.

 

They would then check the inbox for any sites that respond, and they can then reset any passwords, and gain access to any and all of your information, locking you out of your accounts at the same time.