Causes: There are multiple potential causes for not being able to power on VMs:
Problems with file locks
VM file corruption
VM snapshot problems
VM guest OS problems
VM configuration problems
ESXi OS problems
vCenter OS problems
Storage Array/Datastore problems
Network problems
Resolution
When a virtual machine fails to power on, a reason may be logged to the vmware.log file for the virtual machine, to the management agent logs, or presented in the client. Review any messages and consider these points:
Creating a new power-on task may fail if another task for the virtual machine or other component is already in progress, and multiple concurrent tasks on the object are not permitted. For more information, see:
The virtual machine may be configured to reserve physical memory on the host, but the host memory is over-committed and the required memory is unavailable. For more information, see:
The virtual machine may be starting in a VMware High Availability cluster with strict admission control enabled, and there are insufficient resources to guarantee failover for all virtual machines. For more information, see:
The virtual machine may have been previously suspended and making use of CPU features which are unavailable or incompatible with the CPU features available on this host. The virtual machine cannot be started without the required features. For more information, see:
The virtual machine may require both a VT-capable CPU and the VT feature to be enabled in the host system's BIOS. This is true for all 64-bit virtual machines. If the VT feature is unavailable, the virtual machine may produce the message msg.cpuid.noLongmode. For more information, see Enabling VT on Intel EM64T Systems for ESX Server 3 (3282933).
The virtual machine may require another CPU feature which is unavailable on this host. The virtual machine may produce a message similar to msg.cpuid.<FeatureName>, identifying the specific feature it has been configured to require. Move the virtual machine back to the host which has the required CPU features, or edit the virtual machine's configuration to remove the requirement.
The virtual machine may start, but quickly fail with an error during startup. Review the contents of the vmware.log file in the virtual machine's directory for any errors or warnings, and search the Knowledge Base for the error or warning. Base your troubleshooting on the specific messages seen in the logs. For more information, see: