[penguin3107]

Passwords don't just change on their own. It's simply not possible.
A few ideas:
1. You're on a BES and your admin changed your password.
2. Your keyboard is damaged and what you think you are typing isn't actually what's being typed.
3. Someone knew your password and changed it manually on the device.
4. You forgot your password.
5. You never knew the password and this is a strange attempt at finding a way to circumvent it.

Regardless of the reason, you have one more attempt to enter it correctly.
If entered incorrectly 10 times, the device will wipe itself and you can then set a new password.
You'll then need to restore your data from your most recent backup.