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If you cannot get access to take apart the drain from underneath, which I assume is the case, then you could try to use one of two tools, depending on the type of drain that you have. One is a long slender, flexible magnet (far a metal drain) that you can fish down the opening where the trip lever cover is. The other is similar except it has a three prong retractable claw on the end of it. You might find one at the home stores. Good luck

Just thought of another idea: a coat hanger! Bend a small hook on the end and try to fish it down inside the drain........Could work.

Remove The Screws from The Overflow and remove that cover plate. Then Go To Access Panel And Reomve Top section of waste&Overflow Barrel Should Be Right There.

try a strong shop vac and a magnet.