Car wont start - help!

[tamo300]

Hello,
I've been a Yaris driver for three years - ive never had a problem with my car - and it has been so very reliable - however, on Saturday I tried to start my car and it wouldn't start whihc I thought was very odd given that nothing was left on -...so i left it and after an hour I tried to start again and bingo it started as per normal - i thought very odd given that an hour ago it wouldn't start - anyway, being not so car savy I thought it was now working and so after 2 minutes switched it off (should I have left the engine running for at leat 10 mins?) and left the car until Sunday morning - sunday morning i go to the car to do my shopping and it was completely dead - nothing - no sound, not even a choke - just completely dead - like there is now connection - not even a sound of it trying to turn over

I am a bit baffled as to why it would first not start, then after 1 hour start then after a day not start -

The radio is still working as are the lights (all lights) so could it still be a flat or dead battery or something to do with a fuse or immobiliser (ive no idea about fuses or immobilisers)..oh and the wipers are working fine too......

..i have no idea what to do - should i go an buy a power charge battery pack to charge the battery?......I can't jump start it as it's not parked on a hill.....any ideas would be greatly apprecited....thank you