Hello,
By the description, it seems something happened to your local user account and/or system drive. Bitlocker usually only triggers following a mayor hardware or software change, especially if it was an unauthorized attempt. That could even happen from interrupted updates, or a user account credential change.
It's too soon to tell if your drive is corrupted. Try to access the recovery mode, using an external windows boot image. In recovery mode you can run disk repair commands like <sfc /scannow>.
If these options fail and you can't pinpoint the cause of the login change or the BitLocker being triggered, you could try changing the password of the local user while in recovery mode. I suggest you research about this yourself, to decide if you are willing to attempt it. Otherwise, your best next move would be to seek out data recovery professionals. As long as you can decrypt the BitLocker, the data can be retrieved by accessing the drive.