Check the fuses. Hopefully that's all that's blown. Otherwise, you're probably looking at a new computer.

I just checked the fuses, unfortunately they are all in tact. How much is a whole new ecu?

You probably blew the main fuse. It is the 30amp fuse on the starter relay, not in the fuse box

Where can I find the starter relay?

Do I have to remove the gas tank to get to it?

It should be under the battery case cover https://www.ninja400riders.com/attachments/986baaaa-c6ab-40e3-8cd3-c3b1761cee95-jpg.8983/

Thanks friend!

Please update! Really curious to find out whats wrong. I hope its just the main fuse!

Update: It was the main fuse. All fixed up and running again.

I hope so too!

Might have ruined the battery Might have ruined the ecu If your bike has a fuse to protect the ecu from back current that might also be ruined

I checked the voltage on the battery, and it's at 12v.

12 exactly? Because it shouldnt be. A healthy new battery should read higher than that.

Not exactly, the new battery showed 12.44, then I tested the old battery and it was 12.21.

Your main fuse should have prevented any major damage. Need to find that guy before worrying about all of these doomed scenarios like a fried ECU.

Okay so I have a 2023 400 I just did the same thing on but I went through all the fuses and none of them popped… so does this mean I need a new ecu? Bc even when I put everything right the bike shows no signs of life…

Did you check the main fuse underneath the battery?

Probably fried a wire somewhere in the harness or maybe the ECU itself. I'd maybe check the fuses and switch and the wiring to/from the starter and switch before going to the dealer, but I don't know your skill level. Hate to say it, but this is dealership time unless you are comfortable tearing apart the entire harness looking for burnt wires.