I have a 2001 DT125R, which is just under 5K and kept indoors. Looks like new, but last year,.. I could not get it to turn over. Skipped riding last summer due to lack of time to work on it. Was doing some cleaning and decided to spend some time. Guess what, started right up!
Then, it pissed gas all over me and my floor. Soaked it up and no gas left. It had been a couple years since it started... so I figured it needed a good rinse, even with the old gas.
Couple of hours later, new battery and a little more gas later and it started up with no issues. Not sure if it is related to your issue, but I am thinking it was well flooded last year... backed up even.
I suggest letting it all drain. put in a little more gas in a few hours and try starting it up again. Worked for me today.