Could sound weird but verify that you have Intel VT enabled on that host in the BIOS.
Next what setting is under CPU/MMU virtualization for that virtual machine and what OS type is chosen under General settings...?
What hardware version that VM is?
At least remove VM from directory and add it back if this doesn't help remove it again...create brand new VM and point wizard to existing disks...check if you are able to power it on then...