Startup disk of High Sierra disappeared after trying to dual boot with Big Sur.

I am using a MacBook Pro 2015 model. I had macOS High Sierra installed in my machine. I upgraded it to Mojave upon release but downgraded it back to High Sierra since Mojave was causing excessive battery drainage and continued using High Sierra till now. I never bothered to update my macOS until the release of Big Sur. I liked the fresh new look of Big Sur and planned to finally update my macOS. However, I was also worried about hardware issues like excessive battery drainage that I faced during my last update to Mojave. As a result, I decided to create a different partition and install Big Sur on that partition. However, after installing Big Sur in a different partition, the drive consisting of High Sierra completely disappeared. I could only boot into Big Sur but not into High Sierra. I opened disk utility and it can not detect the partition taken up by High Sierra and it identifies it as unknown used space. As such, I neither can boot into my High Sierra not can I access the drive and the data. I had important data into that drive which is now inaccessible. Is there any way, I can access that drive or boot into High Sierra? FYI, High Sierra was FileVault protected. Can it be a reason to restrict access to that drive?