Harrassing calls from creditors regarding Great Plains loan

I received a loan from Great Plains in 2012. Started paying on it but then had to stop due to financial reasons. Now I'm getting harassing calls from creditors. One in particular creditor called today from Credit Solutions legal process department. Said they were going to file a Complaint in my state (Georgia) and have the Sheriff serve me documents and that I had to go to Court next week to pay the loan or risk being jailed. Are they allowed to do that? I contacted Great Plains and they advised me that they don't have a record of me in the system! ??? So how did the Credit Solutions company get ALL my information??? What can I do????