Frigidaire microwave key shows a lock and no keys work

Here is a previous answer that might be of help.......

The first thing I would suggest is a hard reset. Unplug the unit for a minute or so. Then plug it back in, set the clock and try again.

If it is still locked or goes into lock mode by itself, the "clear" key on your keypad may be shorting.
Here's how the child lock works on model FMV145:

Child Lock-Out
You may lock the control panel to prevent
the microwave from being accidentally
started or used by children.
To lock or unlock the controls, press and
hold CLEAR/OFF for about three
seconds. When the control panel is
locked, an "L" will be displayed to the
extreme right.