If you have access to an oscilloscope and make some breakout leads between the encoder and the driver you might be able to check what is happening. By this I mean intercept the encoder wires while it is connected to the driver. I would guess there is at least power, ground and probably 2 pulse trains (slightly out of sync with each other depending on the direction of rotation). Maybe a google search will tell you the pin outs, plus if it is incremental or absolute encoder.

Or perhaps you could figure out the wiring by arranging a breakout connection from a known working encoder to a driver. But this would only confirm that it had stopped working. Then it’s a decision on whether to buy another motor with encoder, or hunt around for a compatible encoder if they are sold separately.

Without a scope, assuming you know which are the pulse train pins and ground you could connect a multimeter and check the RMS Voltage of a pulse train at different motor speeds. The values would not mean much but a reading would increase or decrease with speed.