Late last year I bought an Aprillia Tuareg (600cc Rotax engine). The seller started it up in his garage and it fired up really easily (from cold).
I brought it home, and started it up a couple of days later just to run it up the road, check gears etc. Again it started really nicely. After this I put it away, because I didn't need it over the winter (was running my R1200GS and R1100GS for commuting, and my KTM 400exc and 640 LC4 for really rough weather, local running round, green lanes etc.).
A couple of months later I pulled it out, checked the battery had charge and tried to fire it up, intending to run it up the road for an MOT. Turned it over for a good 10 minutes on full choke but it didn't want to know. I then charged the battery overnight and tried again. Again the bike turned and turned but wouldn't start. I put it away when the battery went flat, confused and annoyed, started revising my plans (don't want to commute over the summer on a bike that won't start).
Long story short, I pulled it out again this afternoon and charged the battery up, and in the sunshine I noticed the faded writing on the choke lever. It turns out the choke on the Aprillia works the opposite way to every other bike I have with a handlebar mounted choke lever! Pushed the choke lever forward and the engine started first prod of the button.
The moral of this story is, if it used to start and doesn't start now, look for the simple answer first!