If the system is still posting you will need to change the Installed CPU to the other available Socket.
Some M'Boards will not work without a CPU 1 and others need the first CPU to be installed in the CPU 2 Socket.
If you pulled a CPU while the system was running you've probably done untold damage to the M'Board and defiantly fried the CPU that you pulled out as these die extremely quickly when the Heat Sink is removed without adding the removal of power and running Data Streams while running.
If the system is not POSTing remove the remaining CPU and fit it to the other Socket and see if that helps. If it makes no difference you've probably destroyed the M'Board and it will need replacing along with at least 1 CPU if not both depending on which Brand/Model of CPU you have installed.
This practise will not have done the RAM any good either so that may need replacing as well.
Col