You can pry the key off and look down the hole, it uses little rubber plungers to hold the key up and depress. see if the thing is crooked or folded in on itself. otherwise, your option is taking the keyboard apart, as moisture got in between the layers of membrane... tho if you want to try, membrane keyboards are water friendly when disconnected-- rinse it in distilled water for a while and then let it dry for a couple of days. there's no guarantee that would work, tho...
membrane keyboards are cool but hard to fix once they stop working properly...
does your other shift key work?
pnw