I recommend coming up with a contingency plan for your worst case breakdown scenario before you actually need it. Some things that could help would be:

-keep the phone number of a taxi service in your cell phone
-know where the closest train stations (if you're lucky) or bus stops are on your route
-a repair kit to handle common problems
-know where bike shops are along your route

I once walked an extra kilometer or so in the rain to what I thought was the closest train station.