Passwords to accounts don't change themselves unless there is a trigger like account-theft protection, manual override, theft, security maintenance, keyloggers, etc. If you've installed a third party application on your computer and it was infected, then most likely your account is already stolen without a chance of recovery.
In MultiBit, changing password is done using the old password for the wallet. Unfortunately, the developers don't provide support for accounts passwords.
Scan your computer with an anti-malware/anti-virus to see if it's infected. In case it isn't, check the Capslock key and reboot the application as well as the computer, then try to manage your wallet again.
