guys pls help ive been tasked with changing the disk controller for an old server, now running on VMware ESXi. everything looked fine until i pwoered it on:

Reboot and Select proper Boot device
or Insert Boot Media in selected Boot device and press a key
No bootable device -- insert boot disk and press any key
The virtual disk is attached and the right size. The original machine booted fine yesterday. Where did my OS go?
I can see the disk in the VM settings, sooo its definitely attached 馃檹 the original server is still around, powered off, so I can check settings there if needed. what usually goes wrong when moving an OS like this??