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Can you turn the FP-4 Off? if so, then you can Reset the unit using button pushes, which might clear up the problem. But you may be unable to do so.
Instead, try unplugging the unit, for 4 hours. (And disconnect it from everything else, and remove any add-ons.) Then plug in again, turn On, see if the problem is cleared up.
After that, a Factory Reset power-up, in which you turn the device On while holding down the Function button for at least until 'Fct' appears on the display, will clear all custom setting out of the device, but might restore functioning.
Note: do not turn the device Off during the Factory Reset.