The battery leakage has destroyed the circuit board.
Beware of Leaking Batteries!
Having batteries that save the presets inside your gear is convenient, but those batteries can also be a ticking time bomb. Eventually, they can start leaking—yet still produce enough voltage to keep your presets alive, so you aren’t aware there’s a problem. But once the batteries start leaking onto a circuit board, they can destroy it to where it cannot be repaired. Sometimes you can solder jumper wires across the damaged sections, but if so, you're lucky. In most cases, your digital synth or signal processor from the 90s (and sometimes even later) will become a doorstop.
So, check the batteries in your gear for signs of leakage every year or so. The process is slow, so you can often see leakage before it drips down on the circuit board. Also, if you’re not going to be using a piece of battery-powered gear (like an effects box, hand-held recorder, etc.) for a while, remove the batteries prior to storage. You’ll be glad you did!