Blue Screen of Death (BSOD) error on the VMs running on ESXi 5.1

Its strange for me, first time i have came across VMs with BSOD (Blue screen of Death) error on the VM screen.

We are running with ESXi 5.1 with Dell R720 and their is no any hardware change on the server. Am not sure why all of a sudden BSOD error comes.

VMs was running with Windows 7 , with 4 GB RAM and 2 Processor

The error on the(BSOD) was

"irql_not_less_or_equal"

Does any one came across such issue, if yes please spread the resolution message you did.

Thanks a ton in Avance