If you have not changed your password and/or ID at eBay there is one other possibility.
If someone tries to hack into your account at eBay, they add an additional layer of security that AS can't handle. After you enter ID and password at eBay, you are asked an additional security question. It may be hard to tell if this is what happened, because (as I understand it) this is reset every night. Try signing into eBay and see if that works OK.
Come back to AS.
Click My Snipes --> Modify Account --> Enter and verify password.
If ID and password does not match AS will tell you, If it does match you will be taken to My Snipes and you should be good to go.