EBay should be platform agnostic. You might have to re-enter your logon and password, but the computer shut down wouldn’t have disabled your account with them. While it is possible your account was hacked, your computer didn’t ‘close the account’.
As for surfing, why do you need an account for that? (Your internet browser obviously works since you are logged on here).
Do you use a password manager that malfunctioned? Do you mean that a user id has disappeared, and you can only log on to your computer as administrator?
As Mr. Guy noted, try going to an older restore point before the first problem.
I’m using my wife’s account. three accounts are on page with icons to identify. Was not set up with a password to enter the separate accounts. Nothing but icons on page, no log on, which comes on next page after I click on specific account. BUT the message comes on at the icon page, with nothing else I can click except to acknowledge the message, which then sends me back to the icon page. She uses Yahoo, I use msn outlook