You have been locked out of your HP tablet or mobile computer
When turning it on the screen that appears is asking for a Bitlocker key
You have never heard of Bitlocker before and you do not know this key, therefore you cannot start your computer
BitLocker is full disk encryption software created by Microsoft software. It is designed to protect your PC against offline attacks and is intended to add an additional level of security, ensuring that only the authorized owner of the computer can unlock the device and restore access, typically if your device gets stolen. More information can be found here:
BitLocker was activated by someone and during the PC activation time it prompts the user to save/store the key in a safe place.
If this is a company owned asset/device, you should turn to your company's IT support guys and they should be able to provide you with the recovery key - search for the PC name in Active Directory and check its properties.
If this is your own personal device, and if you are not aware of Bitlocker key, most probably you use a slightly different BitLocker version for certain PCs and tablets known as Device encryption. Probably your key is stored in the Microsoft Cloud - OneDrive. It should be accessible with your Microsoft account here >> https://onedrive.live.com/recoverykey
Use another secure computer or a smartphone to open the link.
Please, note that it is very important for security reasons that you keep the encryption key (Bitlocker key) in a secure place so that only you know it. Once you locate it, enter it to unlock your device.
Unfortunately, if you can't find your key, or your key does not display on the Microsoft account page, you'll need to restore the computer or tablet to factory default settings, i.e., start over