3.0 ghz core 2 duo shows no signal and core2duo 2.1 ghz plays

I have dell optiplex 780MT. when new buy it comes with 3.0 core2 duo processor working fine. But now problem cames when i start the pc it alwz show no signal. Nd i chnge the same processor to confirm if mine 1 is dead. But no 3.0 core 2 duo it supports.now its only supports 2.1 ghz processor. What d problem. Rpl asap