My XR650L wouldn't start after standing for two weeks. It ended up being dirty (not visible, though) battery terminals!
I made the mistake of reading online and jumping to conclusions too soon, instead of taking my time and diagnosing/testing the issue properly and patiently. Once I started slowing down and using common sense (and a multimeter), I quickly found to problem.
If you've not seen the video yet, I explain the process here: