Why does my truck lose power when you hit the key to start it?

Still the 1988 Ford F150, 5.0, AOD, 2 wheel drive, i had left it sitting for about 3 weeks because the idle air control was dirty and i was supposed to get a guy to clean it, but he never showed up so i went to start it and when i opened the door the overhead light came on and when i turned the key the dashboard lights came on but when i went to crank it everything went out, i thought corroded battery terminals but they aren't corroded as far as i can see (haven't took the cable ends off yet to have a look but there's no corrosion on the tops of the terminals at all) … I feel stupid for asking this but I'm not very mechanically inclined… Haven't hooked a battery charger to it yet because I'm hoping it's not a ground draining my batery power

Added (1). Battery power* not batery

Added (2). Oh and everything had been fine until i just left it sitting for the 3 weeks waiting on someone to show up to clean the I A C for me, didn't want to chance doing it by myself because as i said, i'm not very mechanically inclined

Battery has no juice. Get it charged and load tested.

A) You're not very mechanically inclined? THAT's a gross understatement if I ever heard one! FAIL

b) This is not rocket science. A vehicle has a battery primarily for the purpose of starting the engine. The starter requires a lot of battery power to get the engine started. The moment you turn the key, the electrical load on the battery increases dramatically. If the battery is weak, or has a poor connection, it can't start the engine and may not even have enough power to turn the engine over. Once an engine starts, the alternator takes over, supplying power to the rest of the car's systems and it recharges the battery.

c) So, you might have a weak battery that needs to be replaced, Or, before you parked it you did a lot of short trips in a row and the battery didn't have time to recharge. Or, perhaps, you have loose battery terminals. Or, more likely, there's corrosion under the battery terminals that inhibits the flow of electricity from the battery.

d) Clean the battery posts and terminals. Replace the terminals or battery cables if necessary, Replace the battery if necessary. Make sure the alternator is working.

e) English lesson: Ypou should have asked "Why does my truck lose power when I hit the key to start it?", not "when you hit the key to start it". We're not starting your truck, YOU are dumbass. FAIL