It happened when I "stepped on it" (on an empty residential street). It left the chain on the road.
What tool(s) and parts should I have had with me, to fix it?
It was replaced (new) two months ago, since when they also had the back wheel off to replace its brake pads.
When I lubed it recently I noticed that one of the links seemed loose, but I didn't want to take the time to take it back to the LBS (which is now on reduced less convenient winter hours).
Also, what should I know about buying and maintaining a chain (apart from lubing it and replacing when it 'stretches')? Do you keep a spare chain, or a spare link?