If a USB keyboard is not working properly after a while then there could be a few things to look at.
Firstly make sure all keyboard drivers are up to date.
A second possibility is you are using a key combination for this to start.
Third there might have been more parts harmed than just the keyboard.
One question I have is how did you dry it out?
As for getting it to power on again try pressing all of the keys and see if any are sticking. Then try booting again.
OS: Windows 7 64-bit Case: Commander MS-I
Mother Board: GA-880GM-UD2H CPU: AMD PhenomII X4 965 3.4GHz(Stock) RAM: 16GB GPU: GeForce GTX 550 Ti