Schwinn sidewinder 26 inch bicycle, 21 speed gear, shifters, don’t wor

This Schwinn sidewinder mountain bike has 21 speeds and SunTour shocks and and 18 inch frame. The handlebar gear shifters are stuck so it only rides in one gear. I read on the Internet that you can open them up and clean them and that might fix it or take it to Richardson bike Mart. it has a kickstand and cushioned seat. Location is the city of Heath on Lake Ray Hubbard. If you call leave a message the ringer is turned off.