Stepper motors are entirely dependent on signal timing. When they rock back and forth, it’s almost always an indication that either the timing is bad or some of the signals aren’t getting through at all. I gather you had initial success with it which would mean the stepper motor control cable is wired in the correct order, so that eliminates one possibility.
Check that the stepper motor cable is plugged in properly at both ends and hasn’t just been pulled loose. Direct drive systems, where the extruder motor moves with the carriage, are particularly vulnerable to the cable getting snagged on something. It’s the same thing that causes layer shifting. It’s possible the cable has been partially unseated.
If you’re sure the cable is securely plugged in at both ends, try plugging the Y-Axis cable (or X-axis, depending on what will reach) into the extruder and then operating the extruder by using move commands for the Y-Axis on the controller. As you change the Y-Axis settings, the extruder should move instead. If the extruder does not move when connected to the Y-Axis cable, your extruder motor is likely defective.
If the extruder motor can be controlled via the Y-Axis cable, try plugging the extruder cable into the Y-Axis motor and moving the Y-axis by using the control panel controls for the extruder. I’m guessing it won’t work, in which case, take a very close look at the cable ends. Make sure all the pins are correctly seated in the connectors at both ends.
If the pins look OK, and since you now have the controller board exposed (so you could check the extruder cable at that end), try replacing the extruder cable with the Y-Axis cable. By that, I mean, plug one end of the Y-Axis cable into the extruder port on the controller board and the other into the Y-Axis motor (which we know to be good). That means we have a known-good cable and a known-good motor, so we’re testing the extruder driver on the controller board. Power it up and try using the extruder controls on the control panel. That should move the Y-Axis. If it doesn’t move, you have a problem with the controller board. If your motherboard has individual, removable, stepper motor drivers (which I don’t think the stock Ender 5 Plus does), try swapping the Y-Axis and extruder stepper motor drivers and re-test using the extruder controls on the control panel.
Let us know what you find out.