Need and advice with respect to USB power usage.

Hello. I have an iMac desktop. Recently, my kid tried to charge a helicopter battery using on of the USB ports on the computer. This led to me getting the following message -"Because USB device was drawing too much power from you computer one or more USB devices have been disabled'. I turned the computer off, disconnected the device and restarted the system. Everything seems to work, but the keyboard. And since the computer is password protected I cannot go beyond the start up screen.

I've tried the procedure for resetting PRAM as described here (http://support.apple.com/kb/ht1379), but nothing happens as I suspect the computer refuses to see the keyboard and on every new startup produces the same error message.

I've tried to unplug the computer, same result.

I shall be eternally grateful if anyone can advise me how to deal with my problem.

Many thanks in advance for any assistance you can render me in this matter.