Hi. Last night when I tried to use our tub, I found that the trip lever that moves the plunger closed will not move. After checking several sites, I found out that we have a plunger type of drain. It have the flat cover on the tub drain with no moving part showing in the tub itself. What can cause the lever to stop moving and will I need to replace the whole insides or is it something small.

Disassemble the cover where the lever is and remove the cover and lever rods. Is it brass or plastic? It may need cleaning if brass and it may need adjusting if plastic. If it is stuck and will not pull out, write back, do not pull too hard and break it off in the pipe.