I washed everything off under the hood of my 90 Honda Accord LX now it won't start?

I just purchased the car and it seems as though there was a oil leak at one point because everything was caked in grim and oil from the engine to the under carriage. I washed everything off under the hood with my house hose and baking soda a rag and tooth brush for hard to reach areas. I scrubbed the engine because it was black due to oil burning on it over a long period of time. It's now shiny and looks almost new but this morning I went to start the car and nothing. My neighbor came and gave me a jump it started right up but the second the jump cable was removed it just shut right back off tried again this time leaving cables on a bit longer but same thing it turned right back off with removal of cable. What could be wrong? Also my battery light is now on

Hjhj

Clean the battery posts and the insides of the clamps with sand paper. The battery may be alright but it is not well connected to the car.
Then boost start it again.
If the battery light stays on you may have gotten the alternator wet.
If it will run and get hot it may dry out and be OK.
Otherwise you must have ruined the alternator.
When I purchase a car I buy a new battery.
Seems you will need a new alternator and battery.
(You can get several years out of them, but they are only $100 and so important!) An alternator is $200 or $300. Maybe it wants new brakes and tires too before a new stereo.

You fried the alternator. Water and electricity don't play well together.

Would you rather have a dirty running engine.
or
A nice shiny non runner?
All that washing has got something wet, you now have to find what is wet and either dry it out or replace it.

You got water where it shouldnt be. Your not suppose to get water in the connectors or other electrical parts.