Nexus S won't turn on HELP!
You need the binary image of recovery and ROM 
Now start in fastboot mode
Flash the recovery
Then flash the ROM 


The recovery and ROM can found on manufacturer's website. However, it is a good idea to try to claim the warranty as it's still 2 months, though I think they will not fix it as you have rooted your device.

Issue the following commands on the terminal:

fastboot devices
fastboot flash recovery recovery.img
fastboot flash system rom.img
fastboot flash reboot