If you boot into the bootloader (by holding volume down + power) does it say S-ON or S-OFF at the top? If you're S-OFF, you can flash a custom recovery by downloading a PC36IMG.zip file of it and placing it on the root of your SD card. If you're S-ON, you'll have to either use an RUU utility or an RUU zip file to flash back to stock
f you're S-ON - reference Supersonic Shipped ROMs - xda-developers
Download the PC36IMG_SuperSonic_S_Sprint_WWE_3.70.651.1_Radio_2 .15.00.11.19_NV_1.90_release_161482_signed.zip
Rename this zip file to PC36IMG.zip and place it on the root of your SD card.
Boot into the bootloader (power + volume down)
It should automatically see this file and ask if you would like to update.
This should return you to stock. Warning: You will not have root.