i used my sprint iphone 4 on net10 and now it wont work on sprint anymore

I have an iphone 4 from Sprint and since I upgraded I let a friend use it as a pay-as-you-go phone with Net10. I have the phone back now and I need to use it with Sprint again but they are saying they can't find the MEID number in their computer. First they said it was being used by one of the numbers on our account, but they could see that number has an iphone 5c and now they can't seem to reactivate my iphone 4. What can i do to get my iphone back on my Sprint plan so we can use it again? We are not available to get an upgrade for 7 more months and we need this iphone 4 to work. Any help would be greatly appreciated. It is no longer covered under Apple support.