alt Hacker NewsA BIOS update to my PC reset the TPM only this week. I did get a warning that Bitlocker keys would be wiped as a result before acting at least.
(I believe this was because it was fixing an AMD TPM exploit - presumably updating the TPM code wipes the TPM storage either deliberately or as an inevitable side effect.)
TPMs are basically storing the hashes of various pieces of software, then deterministically generating a key from those. Since the BIOS software changed, that hash changed, and the key it generates is completely new.
If someone had messed with your BIOS maliciously, that's desirable. Unfortunately you messing with your BIOS intentionally also makes the original key pretty much unrecoverable.