I accidentally hit the start stop button during a ride. I didn't notice for a while. can I fill in the missing data after I resume?

Not sure you can :(

Sorry - this is now too late for you, but it's probably worth recording for anyone else who has a similar problem in future. There's no way to get anything from the 1040, of course - the whole point is that it's not recording. 

However, I think it's possible (I haven't tried it myself) to get some data from Livetrack, if it's being used. It won't work for you, because it will have expired after 24 hours, but Livetrack seems to continue recording even when the 1040 is not. There's a thread with helpful (probably - I've not tried it myself) links here - Any way of recovering data from livetrack? - Garmin Connect Web - Mobile Apps & Web - Garmin Forums

Thanks. Was hoping, but seemed like a remote possibility.

Thanks for the thought. I hoped that the info might be recorded in some sort of buffer while the unit was on, but displayed only what was selected. Hopefully I will keep a closer eye on my Garmin in future.

It's fairly easy to do, especially if you're a bit overloaded by other things that are going on. I did it myself for about a kilometre a few months back, but in my case I tend to check the metrics fairly often and noticed before I'd lost a significant chunk of my ride. 

simondaw said:

but Livetrack seems to continue recording even when the 1040 is not.

Yes, but like you said, you have 24 hours to access it even with the option to "Extend Session Visibility." You can use a live track converter, such as LiveTrack export (dsantini.it). I've done this and it does work. Keep in mind that doing it this way, you'll be missing a lot of the metrics you would typically have. You'll have the track on a map, elevation, pace, and heart rate, but not much more.