There are several reasons why this would happen. Here are some six of them:
1). You may have forgotten your PIN. Don’t panic just yet. The “Reset Your Password” option should appear on the screen. BitTruster can also help manage your PIN.
2). An attack might’ve been detected.
3). Don’t press the F8 or F10 keys. This might’ve been useful before, but it’s not on newer versions of Windows.
4). If you inserted a CD or a DVD, then that might’ve also been the problem. This might also break your device, so be careful.
5). Don’t change the Basic Input/Output System (BIOS). This is not an issue on every device, though.
6). Don’t dock or undock a computer when BitLocker is on.