first off, i've power sprayed motors before while they still had the battery in them... only being careful not to spray heavily onto the seals of the engine so as to not break loose any loose seal. electronics can dry and will be fine with very limited corrosion. it's not something i suggest if you're not sure of what will be hurt, but i regularly use a parts cleaner on starters, altenators.. and other engine accessories and they are perfectly fine.
using a damn rag the connection isn't strong enough to unload power from a battery from terminal to terminal.. that requires letting a wrench or screwdriver touch terminals, and it's more likely to drain the battery, not fry the ecu. that's why a car has fuses and relays.. to protect the ecu.
uhhh... if the gas in the tank sat for more than 6 months the gas could have chemically degenerated cause the injectors to clog, starving the engine of fuel.
there is a possibility of shorting something while it's wet, but with a damp rag that's nearly impossible. like i said, i've power washed engines still in the car before and they start right up, only smoking a little while the water and grease sediments that fell onto the exhaust burning off.
check all the fuses, then the relays..
when a battery goes out, there's no sound when turning the ignition on.
when a starter goes out, there's a whining sound as if the starter motor's turning but the flywheel isn't turning due to a stripped starter gear, but for that to happen you would have heard a somewhat loud metal on metal stripping/clicking sound.
when a starter solenoid goes out, the starter won't turn cuz it's an electrical device needing the solenoid to turn it.... easily tested by taking out the starter and checking it at the local autozone. they can test batteries, starters, altenators.
when a fuel pump is operating correctly.. all you do is turn the key to the acc position and you can here it buzzing/clicking by the gas tank.
when a fuel pump is possibly out, you won't hear that.
if an ignition fuse or many other possible fuses are burnt out, it won't start.
a good way to completely determine if the ecu is good or not is to locate it first... usually under a seat or in the passenger's side panel (just to the right of where your feet go.. not in the door but in front of the door) there's an led light on it that will come on when you turn the key on (if the battery is good) that will take out that problem.
but basically you start with all the things that make a car start.
battery-test it
starter-test it
altenator (doesn't really start the engine, just replaces power back into the battery so a battery lasts longer than one drive)-test it, but it's not a main starting factor, just something that would drain the battery and therefore keep it from starting
fuses-check them all
relays-check them too by applying power to the terminals on the bottom of them and you'll here a small clicking noise on the inside.
sensors and stuff aroudn the engine don't really start an engine but certain one do... sensors and solenoids around the engine basically constantly tune your engine's rpm/fuel/air ratios by following commands from the ecu.
the certain sensors that helps start a car are: the MAF (damn... can't remember exactly but it's to do with air flow due to the temperature due to friction of the air entering the intake), the TPS (throttle position sensor... tells the ecu how much air is going in by how far the throttle butterfly valve is open, therefore when to fire the plugs and the fuel injectors to spray), the CPS (crank position sensor... tells the ecu when to fire the spark plugs)
ummm... if the injector solenoids are shorted which is rare due to some water that would have gotten into the spark plug wires or an excess of electrons flowing through the wires... which is constantly regulated by the coil which won't fry cuz it's regulated to increase the volts/amps *this isn't exactly accurate* so the arc can occur across the gap of the plugs.. (it requires more amperage to make the arc than it does to power the coil)... they like to clog up first... then they won't work which clogging occurs from dirty gas... or 6 month old gas being put through them.
hell... you could have knocked loose the spark plug wires.. which wouldn't allow the car to start.
you should go over the entire engine bay making sure you didn't accidentally knock loose a connector on the harness or the spark plug wires... mainly the sensors i just spoke of.
beyond that stuff... (and this is all stuff that is on most cars but in different makes are slightly different and in different places... this is all what i know off my 240) beyond that... i'd wait for some more replies on here.
maybe ask some ppl on here that have a lot of posts or have threads of them modding their engine cuz if they're modding then they already know the basics.