My customer ran a 'yum update' on their VM and after they rebooted the VM, it will not come back up.
All OnApp Versions
To boot the VM, the kernel should be rolled back to the previous version:
Boot the VM in recovery mode via the OnApp UI.
Go the VM console, login as root and the password is recovery.
Once logged in run:
Find the disk for the VM for example ( /dev/sdb1 20GB )
Once you have found the disk we can now mount it.
mount /dev/sdb1 /mnt
Once the disk is mounted successfully you can then go in and edit the bootloader file.
Chances are the default is set to 1, if so try to change to to 0 and boot, or choose the number of the kernel you want to boot. Note, 0 is the top kernel in the list.
Unmount the disk:
Disconnect from console and shutdown the VM from the OnApp UI.
Then boot it up and all going well the VM will boot normally now.
Usually when this happens, the updated kernel is not a Xen kernel, so the VM is unable to boot under Xen. The kernel should be rolled back to the last usable version.