Someone managed to get your serial number and is using your software is pretty much all that can mean, I think. Perhaps a former employee has decided to go into business for themselves and copied the software without you knowing about it and you're now locked out.
The only thing I can think of would be contacting the people you bought the software from to see if they can help or the software developer to see if they have a solution. It's 99% likely that they'll want proof you bought the software (no matter how long ago it was) and 50-50 they'll tell you there's nothing they can do and your only solution is to buy the software again.