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If you use a Microsoft account you may find your bitlocker recovery key here: Bitlocker Recovery Keys
If not and you really have forgotten it (you didn't print it out or save it anywhere) then you are out of luck.
Bitlocker Recovery Keys Frequently Asked Questions
I am not sure of this, but I think the System Images made with Windows Image Utility are not encrypted.
So, I think restoring from a System Image will overcome this.