Somehow I managed to corrupt my android emulators (the google ones). I think it was because I had some emulators defined, then removed most of the platform SDK (for some other unrelated testing), then reinstalled some of the platforms.
The emulator manager said a couple of the emulators were repairable, which I tried, but it seemed they weren't really repaired and would not start. However, from with XS, I couldn't tell. I would click run / debug and XS would flash up that it was starting the emulator and then stop.
It took some time for me to realise that it was actually trying to start the emulator.
What could be nice is if the emulator quits really quickly, that we detect this and display a message. At the moment, it assumes that the emulator was quit because of user intervention and does not display any message and this could be a little confusing for the user.
*** Bug 36836 has been marked as a duplicate of this bug. ***
FYI for myself, it's also possible to trigger this if you delete an emulator but google thinks it's in use and doesn't completely delete the emulator - it leaves the entry for the emulator but deletes the actual emulator. XS still thinks the emulator is there and when we run we get no indication that the emulator didn't run, the play button just turns back to the play button (from the stop button).
Fixed in version 126.96.36.199 (master)
Author: Greg Munn
Commit: ec91995a891c4fe176379e26f7294e3178e84cd1 (xamarin/md-addins)
Included in Commit: baafad5b69f68333b940cc2e78ddd373b907cc60 (mono/monodevelop)
Fixed in version 188.8.131.5226 (cycle8)
Author: Greg Munn
Commit: a0509d60d4d6602601a75e0661de0e5991c69d5d (xamarin/md-addins)
Included in Commit: da80f10c1a9e7de4903c987e67d7f18f0bbc7833 (mono/monodevelop)