Have you checked the battery cables for corrosion? You might try scraping the insides of the clamps and get some new lead exposed and scrape off the battey posts so you get a good connection. Also check the ground wire and see if that didn't come loose somewhere.
If those things are alright try jiggling the key in the ignition cylinder it could be the ignition cylinder where something didn't set right when you turned the car off last nite.
Has your key ever gotten stuck in the ignition and you couldn't get it out?