How do I learn what exactly has been changed when my account has been illegally accessed?

[#1]

I just received an email from Square, telling me that someone in Ontario, Canada has used my email account to access my account!  After changing my password, I sent emails to the Square Team.  How do I find out what, if anything, has been modified on my account?  I think that there should be some immediate source to communicate with for help in the event of an emergency like this.

[#2]

Well hopefully you logged into to the website directly by typing it in your browser and didn't click any links in the email. A lot of emails like this look like they are from square but are actually hackers hoping you click the link in your email brining you to a page that looks exactly like Square and you log in with your valid login information and now they actually have it. 

First thing i would check check is your bank account information to make sure that hasn't been redirected. 

[#3]

@CMS - Thanks for reporting this right away. We have a team at Square that is focused solely on account security. They'll definitely take a closer look at your account. While you wait to hear back from them, I'd strongly suggest that you enable 2-step verification, as well as double check your bank account on file.