I had this exact problem yesterday where the CPU fan came on like a jet engine and the PC didn't start and the monmitor was blank. I simply opened the case, blew out ALL the dust bunnies, in the case and on all the boards and heatsink, fans etc, reseated all the power supply cables/connectors, especially the main pcb fan and 20 pin connector on the main pcb. When I plugged it back in, everything was fine. Yay!
Just clean all the dust from the heatsinks and reseat all the connectors. I did not fool around with the cpu or thermal paste etc.