Thanks all for the advice. Turned out to be something minor - the PTO lever was partially engaged. Once disengaged it fired right up.
Fortunately, I and the machine were pretty much unscathed. Classic conditions for a rollover - very last load at the end of a long day, fading light. Picked up a partial bucket of drainage rock with the FEL, went around to the other side of the tree to fill the other side. Even though the rear wheels were on level ground, the uneven load and the side slope that the front wheels rested on was enough to bring me over - quicker then you might think.
Guess that's what happens when your brain is disengaged...
