You shouldn't be able to disable CPU sockets.  IF, for some reason, it is actually disabled by a setting, clear the NVRAM using the jumpers on the motherboard.  The default setting would be "on". CPU socket 1 MUST be populated, regardless of whether you use one or two CPU's.  That means that if CPU socket 1 stops working, you need to replace the motherboard, since you can't use CPU socket 2 by itself.
Hello everyone ,  Hope you can give me some help with a poweredge 2900 having 2 CUP's sockets but Only 1 CPU , the 1st socket is OFF the server boot but nothing happen , i changed the CPU to socket 2 , the server boot but it halt with the messg : "change CPU from socket CPU 2 to socket  CPU1  " ,  I think there's some config to do but i can't enter system setup when i press F2 , it actually write " entering setup " but the system halt with the message above  Can pls someone give me a hint :) Thank you  
Changing CPU from a socket to another