Paypal Issue very serious?

I have had a very serious and important issue on paypal. I was supposed to get a payment for my service from a particular guy. He paid me thru an email id to my paypal id. After I got the payment, some times later, i got an email from paypal claiming that the payment that i have just received is not valid as the user has not authorized the payment to pay me. What this could mean? Has that person hacked some one else's paypal id and paid me or could be due to other reasons, kindly explain.

I also had another issue where i got payment on 24th decemeber (from a differnt guY) thru paypal and today i got a mail from paypal claiming that that payment was also not authorized to insufficient funds. How could this happen, at least around 6 days later they have found insufficient funds? What should i do on both the cases to resolve my case. Kindly advice please.