Your must use the original gray system install/restore disks that came with the eMac, or a full retail install disk for no higher than Tiger 10.4 (assuming it's a real 2002 model with a 700 or 800 mHz processor).
The gray disks from another Mac model seldom work. The grey disk from an Intel Mac won't work at all on an eMac. Snow Leopard install disks require an Intel Propessor and therefore have no code for a PowerPC Mac.
It's a long shot but, if in N. America, you can call the main Apple store (1-800-MY-APPLE), ask for a system specialists and ask him for a replacement disk set. If they have them, there will be a nomimal charge but far less that retail install disks, which can cost more than a used eMac is worth.
You will need the computer serial number at hand when you call. Here's how to find it:
http://support.apple.com/kb/HT2395