Pad retraction relies on the spring in the master cylinder. Shimano levers seem to be particularly prone to getting bunged up with mud etc, the spring isn’t strong enough to push the stuck piston back.
The servo-wave type levers only push on the master cylinder. Pulling on them does not pull back on the master cylinder.
After pushing the pads back you would usually need to give the lever a few pumps before the pad position is reset. The stuck piston means you can’t get the fluid into the system to allow this, giving you the no brake effect.
You could try stripping the lever and giving it a good clean to hopefully get it moving again.
That or it’s new lever time.