Hi lavagasm-
Did you receive a payment into your balance or did you receive an email notifying you of a payment to your balance? It's a common phishing technique to send a fake notification of payment requesting that you click on a link for more info, which then may download malware onto your system or notify the phishers that there's a valid email address for future phishing attempts. If you clicked on that type of link, I'd suggest changing your password and running an updated security scan on your system.
If you genuinely did see the amount post into your PayPal balance, there should be a transaction details link that may shed some additional light assuming there wasn't a note from the sender included with the payment notification email. If there's no way to get in touch with the party via email, your best bet might simply be to refund the total back with a note requesting further details with a re-sent payment (those details would be up to you based on what you need). If you use the refund from the Transaction Details page you've clicked into, you will also be reimbursed the fees you paid for PayPal to process the transaction.
Thanks,
Jason
