Ideally, you can return it from where you purchased it.

Wait, you said stopped working after you opened it? Why did you open it to begin with?

Nintendo returns are a bit of a hassle but doable, but if they can’t guarantee a 1-1 replacement, you might be on the hook for a swap (I don’t know what this takes to do or feasibility of it), or, may be sell the replacement and use the money to buy an exact replacement online?

I can’t think of any other avenues to get you whole. I love the Switch, but joy-cons are trash.

You probably fried the fuse on the motherboard when you popped the battery with a metal tool.

Well officially you already waived your warranty by opening it yourself. So even sending it in Nintendo might noticed you already did that and won't do anything about your issue.

Doesn't it work at all anymore? Even connected directly to the screen?

Or maybe just the LED's aren't working (and with that likely the Sl and Sr as well) but still making a wireless connection?

And why did you open it in the first place? What was wrong?

Check the pins on the joy-con side and switch side to make sure they aren't damaged or full of junk, try with a friends joy-con if you can to test it's not the Switch tablet. If those look clean, try unplugging and re-plugging the internal battery of the joy-con and try recharging. If that doesn't work and the joy-con motherboard doesn't look damaged from liquid etc, you can try a new battery from Amazon for the joy-con for pretty cheap before calling it quits on that controller.

Source: I've repaired 100s of Switches and joy-cons for my business.

When swapping out the shells for a shell-replacement I accidentally must have nicked a PCB with a screwdriver, too. One was just... dead. Tried everything, no dice.

So it might just be a loss. The things are fragile as hell when open.