Let's say the following is being displayed when attempt to power on one of your virtual machines. This suggests that one of the data store .vmdk files being used by the virtual machine is not found. I had this happen when the data store hard disk drive failed.

Failed  - File /vmfs/volumes/56f4ed2a-0bf6918c-5298-10bf4879952f/vm1.example.com/ds1.vmdk was not found

Enable SSH connections to your ESX hypervisor.

Create a back up copy of the virtual machines .vmx file.

cd /vmfs/volumes/your_datastore/your_vm/
cp foo.vmx foo.vmx.backup

Set present to FALSE for the bad .vmdk file.

scsi0:2.deviceType = "scsi-hardDisk"
scsi0:2.fileName = "/vmfs/volumes/56f4ed2a-0bf6918c-5298-10bf4879952f/vm1.example.com/ds1.vmdk"
scsi0:2.present = "FALSE"