this sounds to me like its the good ol classic SOD (sleep of death) situation so what you need to do is either flash a custom rom, there are many great ones in the development section or flash to the original stock 6.1 rom which is also found in the development section.
EDIT
THIS THREAD will tell u everything u need to know about flashing custom roms and provide what u need to do it and
THIS is the direct link to the stock 6
1 rom that was taken from
THIS THREAD
either of these will solve the issue u are having if it does not then u may have to contact sprint for a replacement under insurance