If you are running vmware ESX 3.5 Update 2 - you may want to check out this news from NeoWin.net. Quoted from the posted article…

“This was first reported yesterday [August 11, 2008] evening on both the VMWare Community forums and on DeployLinux.com. From what anyone can tell, there is a bug in the VMWare License Management code and it is causing any system that is running ESX 3.5U2 to not be able to boot this morning.”  Continue reading the article at the source.

You may also find the post helpful from the vmware community site.