I saw your other post after I replied to this. I don't know anything about the F840BT. But I know that the Z-110BT also has that lock "feature". From the Service Manual:
Quote
If you enter an incorrect password three times, the screen is locked and operations using touch panel keys are disabled.
There's nothing in the service manual on how to recover from this. So you may have to send it to Pioneer to fix. However, if it was me, I'd try manually going into testmode and seeing if there is any file in the USER\SYSTEM\SECURITY directory. However, it is possible that you might not be able to do anything interactively if the hard keys are also locked.
To get into Testmode, download SuperTestMode.ZIP and unzip it.
Put the testmode.key file on a SD, put that in the head unit and it should boot to testmode. If it doesn't, restart your headunit. Note: do not use the Testmode.key file included with my Security Password Removal - that one only runs a script. This "super" testmode key runs in interactive mode.
It is a horrible user interface, but Go into File Maintenance. Then navigate to USER\SYSTEM\SECURITY and see it anything it there. If there is some file, it might be a "lock". So delete it.