Invalid virtual machine

We're using ESXi 4.1 update 2 with the latest patches.

I have a virtual machine that I wanted to revert to a snapshot. Once I did, the virtual machine became "invalid".

I removed it from inventory and tried to add the virtual machine through the datastore browser. When I added, it showed the vm in the list but still invalid.

I checked the data browser and didn't see any lock files either. I renamed the file vmxf and tried again to add but still the vm is invalid.

Any ideas how to solve the problem?

Thanks,

Haim Beyhan