First off, What's a plughole?
Second: try taking a wire coat hanger and untwisting it. It can be turned into a very long, narrow hook that can fit into tight spots.
Added: Better method, if the plumbing is the same as it is in America (Assuming you're living overseas based on "plughole" simply removing the trap should allow you access to the ring. The trap is the bit of pipe under the sink that drops down and then comes back up to connect to the piping coming out of the wall under the sink. (called a "P trap" I believe?) You can do it yourself, but you'll want to make sure you've got some sealant and other supplies to put the trap back. Goto your local hardware store, explain what you're going to do, and they should be able to help you out a great deal more then anyone here on the internet. (Having access to actual parts to show you what you're gonna do.) Good luck. ~MisouAngel