Did you check the BIOS Setup (F2), CPU/processor options to see if you need to re-enable VT?  Could have been disabled with installation of a new processor.  Also, BIOS needs to be at the latest (as well as ESM/BMC and PERC).
power edge 1950 IIILast version BiosCPU : E5345Windows server 64b 2008 HYPER-V is running !!! Followed by upgrade CPU for E5420 Server windows 2008 is running   but Hyper-V not running (CPU problem) What is the problem ?
PE update CPU hyper-v