How to unbrick/reflash Nexus S using stock ROM, stock tools
You should try a factory reset before trying to reflash the phone.  AFAIK you need to unlock the bootloader (voiding your warranty) to flash a ROM, even on the Nexus S.

Instructions:


  
  Turn the power off. If your Nexus S is frozen, pull the battery out and reinsert it
  Hold the Volume Down button
  
  Press and release the Power button
  You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
  
  Select Clear Storage by pressing the Volume Down button
  Press and release the Power button
  Now simply confirm your decision: Volume Up for YES and Volume Down for NO