Hi!
If your Amiga 500 worked fine before putting it back to its box, years ago, it's unlikely that a component failed "itself" completely.
The much probable cause of this failure could be solderings and and, even more, bad contacts of the CIA chips with their sockets (it happened something similar to me too).
Depends also on the environmental conditions you put your Amiga.
Anyway, the CIA chips (labelled 8520) are placed in the central-upper part of the Amiga 500 motherboard (more or less in all revisions); on the motherboad is written ODD CIA, EVEN CIA.
To remove them use a little screwdriver (if you don't have a proper tool), forcing the chip to come up from one side and from the other.
With some patience you can remove them easily.
After this, to clean the pins (_be careful_), scratch them _GENTLY_ on a piece of white paper, in the outer part and in the inner part: it will get grey for sure (the oxidation).
Replace both chips in place.
This applies if no bad signs of oxidation are in place due to a leaking battery, for example (I assume you have a plain A500 not a Plus, with an internal battery).
If you have an expansion with real time clock, I hope it's closed (eg. A501) or the battery is far enough from the motherboard (if bad conditions, leaking acid, remove it!).
After this, the problem could be related to the floppy drive too: check if the drive makes a small audible "click" regulary.
If so, it could be a electromechanical problem inside the drive, in the contact that let the computer know a disk is inside.
In this case you should open the floppy drive.
I have a similar problem in my A600HD: I still have to solve it, but I have to find time to do it.
Hope this quite long explanation helps.
Ciao!
