Believe it or not I had the very same situation. Took it in to my mechanic (once I got it started) and he checked all the connections and they all seemed fine. Went to work and after work same deal-wouldn't start. Had one of the boys give me a boost and that didn't work either-but I noticed the dash lights did not come on, while he was boosting? Let it sit for awhile and then tried but now there was no power at all when I turned the key. Buddy put a wrench to the positive battery cable and boom instant power. Could not believe the terminal was that bad, as it looked good with very little corrosion. Drove it home and cleaned up the terminal (cold and dark out) and called it a day. About three months later the same thing happened again. I was off that day, so I put a wrench to it and managed to make a connection and backed it into the garage (first time was winter and now was summer). I removed the terminal and the red boot that protects the terminal to see how much corrosion there was buried. Really not that much. Super cleaned the copper cable end by dunking in vinegar and salt solution (restores to original condition) and contact cleaner. Cleaned the both negative and positive cable terminals (side mounts are not my favorites) and wire brushed the contact surfaces on the battery and the terminal bolts. Put the red boot back on (that was a bit of a struggle, but with some silicone spray and Heat Gun I finally got it re-assembled) bolted it up and sealed with Dielectric grease.
Has been rock solid last 4 months.