If your encryption key is incorrectly applied, the content is not accessible by the boot system. You need to find a way to apply the correct decryption key to your drive, the one that was applied before the upgrade.
-Update-
Before attempting boot fix check in the BIOS to see if the HDD access mode did not switch between IDE/SATA/AHCI/RAID. Use the original setting and it should boot. If that did not changed, and if you are sure the drive is decrypted do the following:
Boot to the Windows DVD, Select Next, Repair your computer
Select Next, Command Prompt and to X:\sources\recovery, then type StartRep.exe to launch the automated startup repair utility. This must be repeated if it fails the first time.
If it still fails, open the command prompt and type bcdedit. This will display the values for Windows Boot Manager. Look for Device. If it is set to unknown, type the following commands (assuming C: is your boot drive)
bcdedit /set {DEFAULT.EN_US} device partition=c:
bcdedit /set {DEFAULT.EN_US} osdevice partition=c:
bcdedit /set {BOOTMGR.EN_US} device partition=c:
Reboot while removing the disk and system should boot fine.
If it does, you can re-encrypt your drive. Keep everything MBR+Legacy.
But the drive is already decrypted, if I run their recovery tool it says it's not encrypted, I also don't get the pre-boot any longer. – cyzczy – 2017-01-31T12:43:38.720
You can access content normally (if you connect it to an external SATA-USB adapter) ? – Overmind – 2017-01-31T12:51:48.083