Why wouldn't my car start?

[Stag>75]

First time this has happened to me. R75 2.5 KV6. Drop about 8 km without issues and parked the car. The road was at quite an angle to the kerb so the car was on a tilt to the left. I came back to it about 1/2 hr later and it turned over fine but would not fire. I tried a few times but to no avail. I lifted the bonnet to determine if there was anything obvious but didn't have any tools with me. I wasn't able to tell if it was fuel or spark related. After leaving it for about 5 minutes it started fine.

My concern is that it might happen again in a less convenient place. Any likely candidates?