Do phones have a power supply? This is definitely a noob question, I'm not an engineer but as far as my knowledge goes, the power supply on a phone would be its battery.  However, my Nexus One stopped turning on recently, the battery charges fine as the LED indicator shows, but when I try to turn it on I get nothing.  This is the result of me taking the phone apart and trying to clean the contacts of the sim slot as it wasn't recognizing sim cards, and after putting it back together I can't seem to turn it on anymore.

I then took the phone to a repair center and the guy told me the power supply is bad that he would have to replace it, the battery is fine but its not getting power to the device.  So my question is, what is this power supply he speaks of?  The battery is fine, and hes obviously not talking about the power adapter... and what could cause this power supply to fail?  I followed the directions here to take apart
Power Supply is actually a term with some actual, albeit generic, significance and meaning. 

In handsets, the power supply is probably(I'm not an electrical engineer, or expert on device hardware) a component that regulates the power supplied to the device (keeping the voltage constant, for one thing), and manages the physical charging/discharging of the battery.

As far as what could have damaged it, it could be anything from dropping/shaking the device if there is a loose connection, to a short caused by water, to overheating. Unless you can find a connection that got loose, and the power supply is indeed fried, you'll need to get it repaired.

If you think there might have been any water damage, put the phone in rice for a day or three, it will dry out most liquid that may be in the device, and it might just work.