Shine your flashlight down the pipe. Can you see standing water? If yes, you have a clog.
Insert the plumbing snake into the standpipe. Crank the handle while pushing down. You are trying to break up the blockage (usually a mix of lint, soap scum, and fabric fibers). how to unclog washer drain
Twist the knob counter-clockwise slowly. Water will gush out. Let it drain completely into your pan. Shine your flashlight down the pipe