after the server hanged, FANS running in very high RPMs

I upgrade (by replacing)  my Dell Power Edge 2950 from 4GB upto 16GB, (8x2GB), everything works fine. after a month, I re-stared the server and it halted, so I switched off the server, I removed the power cables from the 2 power supply, I removed the RAM, start the server without RAM, shutdown the server again, reinstall the RAMs, when restarting the server it works fine, but the FANS running in very high RPMs.

Any Help

THANKS,,,,,