Can't boot server after HD replacement

This is a bit of a long story, so hold tight.
I have 2 servers which I have been renting from OVH for over 11 years, and last week the inevitable happend - 1 of the 2 HD's failed.
Fortunately the dive which failed wasn't in active use, but it turned out the drive which was had 300+ bad clusters.
So OVH replaced the drive which had failed and I got a friend who's a server admin to set up RAID.
When he was happy that everything was running fine I again contacted support to get the primary drive replaced, and that was the beginning of the nightmare.
So now I have a drive with all the files on from the failing drive, but it won't boot. Support has kindly given me short-term KVM, and they have tried swapping the drives around, changed the bios, all to no avail.
The server was running Windows server installed using a script by a person who was offering that service on here back in the day, but no longer does it.
Is there anybody able to either offer advice, or able help getting my server back up please.
OVH advice is repair the boot loader.
Thanks in advance..