I'm having a similar problem. Mine was because, when I went to Best Buy to upgrade my phone (they were the only place in town who had one left), Sprint's service was down, so he called me later to tell me how to activate it.
There are two things you need to do (as far as I know. I'm still working on it):
1: Call Sprint, have them run through everything, I think there's something they need to activate on their end. If they decide to help you, then they'll tell you about the next step. If not, read on.
2: Dial ##778#, hit "Edit". Put in your lock code. Hit "View Info", click on the "mip setting", go into the menu and hit edit. Change "Active User Profile Index" from 0 to 1. Hit OK, then Save. Your phone should reboot, and you're good.
The caveat, of course is that if Sprint won't do anything on their end (I think they need to activate something on their computer), or if you don't know your lock code, you're screwed.