2007 KLX250S with about 5k miles on it. Started and ran fine 2 days ago.
When parked, the AGM battery is typically on a Battery Tender charger.
Today I went to start it and, when turning on the ignition, I get no electrical power to anything.
With the key off, multi-tester shows 12.51V when connected to the battery.
With the key on, multi-tester shows 0.4V when connected to the battery.

I don't have an accurate owners manual, so where is the fuse box?
Next step is to confirm a good, clean ground to the frame.
After that, where should I start testing circuits and tracing wires?

 

Update: Found main fuses (under seat, in front of rear fender, inside a black hinged plastic box), both are good.

 

U need a new battery.

 

First how old is the battery, and did you check both cable connections for looseness and/or corrosion?

 

All of my connections were clean and tight, I check things like that regularly.
Hooked up my Schumacher charger / starter to it, seems to be the battery. It's a 2-year old Duralast Gold AGM (from Auto Zone), and it died very suddenly with absolutely no signs of weakness.
Local stores didn't have the right one, ordered a new AGM battery (with better warranty and lower price anyway), scheduled to arrive Friday.