Jump up and down and curse.
Then settle down and call the mechanic ;)
I went down to start my car today around 11:00 (EST, now ;), and to my surprise, it did absolutely nothing! No click, no crank, no noise whatsoever. To make matters better, I couldn't get it out of Park either (Automatic). So, I tried some basic things to see if my battery was still good: I turned on my headlights and turned the key. They didn't dim at all, which was a good indication that no power was even being sent to the starter.
Eventually, the CVM Service guy called back, and I told him all that was going on; he was just about to tell me that it would need towed when I told him about the transmission thing. He then told me to push the shifter forward hard, then try to start it. OMG, it worked! Apparently, I had the transmission just enough in Park that the key would turn, but not quite enough to allow me to start the car. Quite interesting (thanks Alan!).
Just in case you're wondering, this was all in my 1997 ('97) Subaru Legacy GT 2.5L Automatic (how's that for some keywords?).