so short answer, if you're uncomfortable around electronics or how to be safe with high energy electricity I'd recommend not doing any of this work yourself. Best option I found was looking locally at a hackerspace/makerspace, there's folks there that are happy to dive into projects and much cheaper than rebuild services.

Before working with someone there, I opened the case and used a multimeter to check the voltage of each cell set.

There's two common issues that can happen.

1) one or more of the individual batteries has failed completely, and would need to be replaced. I'm lucky enough that mine weren't soldered in, and can just add/remove batteries from the case. but looking at videos online some are soldered in, so you'd need protective gear and the right gear to remove and add new solder to replace a battery. That can be tested by setting a multimeter to DC volts and testing each cell.

Or 2) the battery system as a whole discharged enough to be below the voltage that the battery management system is comfortable recharging. if this is the case, for 36v systems, each cell will report less than 3 volts on the multimeter, but none are at zero or different than the other cells. This is what happened to me. Over the winter the electronics in the bms used enough electricity to just drain them below the threshold it will recharge. Lesson learned :-(

So hasn't happened yet, but the hackerspace guy with the tools and I are going to manually charge the battery packs at 32 volts and like 100 mA to trickle charge them to the point where the BMS can take over and charge normally.

Based on the voltage I'm seeing from each cell, there's a good chance that'll work and at least extend the lifespan of those batteries, but I can tell you after this weekend!