droid with Red triangle in recovery

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http://forums.androidcentral.com – hey guys, I am noob to rooting. rooted my nexus s GT-I9020 recently and installed clockworkmod recovery 5(worked finely when installed). tried installing slim bean, installed rom manager also. flashing recovery was un successful and when tried accessing recovery and found the droid symbol with a red triangle in its stomach!!!! I am not sure where i messed it! Kindly help me please. Many thanks!!! (HowTos)