Can’t locate firmware passcode

I have recently found myself in a difficult situation with my MacBook. After putting it in lost mode, my Mac decided to firmware lock. I can’t find or recall making a firmware code or password. I bought this Macbook second hand, pretty much brand new. I took it into an Apple Store, where they said I need to find the original receipt of purchase (which is just impossible to find) as it means tracking down the original owner. The company I bought it from said they don’t have access to getting the original receipt. So this has put me in an awful situation, as I’m soon about to be going into my last year of Alevels, with all my work logged on this laptop, I cannot afford to lose it all at this stage in my education. It would be great if Apple could just unlock it for me, as this seems like the only way. Any help with this would be great

Many thanks,

Theo