If it is not posting chances are the HDD is not the issue. I see if it posts with an external monitor first. If it was in an area that was subject to extreme temperature swings something may have happened to the LCD. This logic board does also have an ATI graphics chip on it which I have problems with in the past. Generally with apples the beeps coincide with flashes from the power LED so if there are no flashes I would say it is not throwing a beep code. If it does not post externally I would take it apart to to expose the BGA chips and try to heat the ATI chip on the board up and see if it posts then. Use a hair dryer or heat gun but do not go over around 175C. You are not trying to reflow the chip just heat the connections under the chip. I have used this method to see if there is a BGA issue quite a few times. You should be able to just short the two points in the connector that the power plugs into to start the laptop up so you do not have to put it all the way back together again. You could also leave it sit for a few hours or even overnight and see if it starts posting again. If this is the issue you could try to reflow the graphics chip or just buy a new logic board. there are some available on ebay for $55 from a vendor that I have used in the past...
http://www.ebay.com/itm/Apple-Ibook-G4-A1133-Logic-Board-820-1832-A-1-33GHZ-/160667326449?pt=Motherboards&hash=item256884d7f1