What's wrong with my car?

Sound like a fuel problem. like the fuel pump was not pumping enough gas to the engine for it to run a t low rpm's, it could also be the fuel filter, when was the last time you had the filter changed?

just because a battery will play a radio ,that doesn't mean its good,a bad battery will play a radio most of the time,but if it started back up,it could have just bogged down and died,this will happen in hot weather,it may have also vapor locked on it,that will shut it down ,kind of like its not getting any gas,sometime they will start back,you can simply get out and remove the gas cap,and then put it back on to make it start back up,as long as the battery will turn the engine over it should start,good luck on it.

I'm not sure becuase it could be a lot of things. You say it just died which suggests to me a possible ignition system problem. You might need to take it to a mechanic just to be on the safe side.

you failed to mention wheather it will crank back up

or how much fuel is in it if there is plenty of fuel it is possible that you umped time how many miles all these things are very important my roomates 02 pt jumped time at abou120k mi. it could be that as you pulled up the hill the fuel in your tank went to the back and starved your engine if this is possible roll back to the bottom of the hill and try to crank it up when it is sitting level

Is it a automatic or standard you may not have given it enough gas or it could be the alternator not to sure

Could be the alternator. When my car did that I had to have the armitchur ( i know i spelled it wrong sorry i don't know how to spell it so i typed it how it sounds) in the altenator rebuilt

is could have been that you remembered how much you had wanted that 06 scion you had seen at a junk yard, so you turned off the motor and started to call up the junk yard but then forgot