If it’s stuck down in the U bend undo it as already described:
If it’s a big cap and it’s sitting just below the plug hole you’ll need to either: poke it up from below (after undoing the u bend) or poke a hole in it from above and hook it out.
If it’s wet it’ll be very hard to make anything stick to it but if you can get a wire coat hanger and undo the twisted ends you can heat the end with a lighter and poke it straight down through the plastic cap. Then twist it so the wiggly end goes in like a corkscrew and pull the cap out.