Need some help!

Bought a 2005 400 a couple weeks ago - has run flawlessly since I bought it, I’ve put well over 100 miles on it already. Did some work on it yesterday - basically just replaced the air filter with K&N. Also put Tribotex and Redline Moto Fuel cleaner in it the day I bought it.

Took it out today and it started up fine but died at the end of my street. I thought I might have hit the kill switch so I flipped it back and it started up just fine - died two more times on the street to my destination - but started back up each time. Finally died and I was able to pull into a parking lot - I was giving it throttle with nothing happening and I heard a loud backfire and now it won’t start.

Obvious things: Battery was replaced when I bought it and it’s turning over fine but won’t start. It has a half tank of fuel, good 87 no ethanol stuff. Side stand isn’t down but the switch is fine, won’t even turn over with the stand down.

I got it home and noticed a few things - what seems like lots of pressure when I undo the gas cap (tried starting it without the cap, didn’t start). There’s an interesting whirring noise when I press and let go of the side stand switch - goes away after a few seconds - sounds like an electric motor, maybe a fan or fuel pump or relay energizing?

So, where to start? It’s odd to me that this kinda came from nowhere, I don’t think the air filter is connected to this at all but I’d supposed it could be. Hoping it’s something simple like a relay or something. I’ve read something about a diagnostic port and jumper and reading codes?

Thanks!