After the upgrade to ESXi 5.x, ESX 4.x resource pool settings might be insufficient to start all virtual machines in the pool.
The upgrade to ESXi 5.x affects the amount of memory available to the host system. As a result, in resource pools that are set to use nearly all of the resources available, some virtual machines might not have enough resources to start after the upgrade. When this happens, a system alert will be issued. You can find this alert by pressing Alt + F11 in the ESXi direct console. Reconfigure the resource pools to solve the problem.