Sounds like one of your keyboard "modifier keys" are stuck. So you're typing Shift+S, but your computer thinks you're typing something like Ctrl+Shift+S.
One way to clear it is to tap on the each of the modifier keys twice. Press Ctrl twice, then the other Ctrl (if there is one on your keyboard) twice. Then the alts, then the shifts, then Escape twice.
Afterward, try typing again. If still a problem, reboot or try cleaning your keyboard. If still a problem, try another keyboard. If you don't have one, a keyboard is about $8 at a place like Walmart.