I have a schwinn ranger suntour mountain bike which I enjoy riding and ride frequently. When the bike was new I had no difficulty shifting through all 21 speeds of the bicycle with 3 gears by the crank and 7 by the rear wheel. I can shift through the 3 gear speeds with no issue, however currently when I attempt to shift to any of my rear 7 gears nothing usually happens I appear to be stuck around 2nd or 3rd gear no matter which gear is selected and I am noticing more frequent chain slipping.
The amount of slack for the cable for the derailleur for those gears can be adjusted by loosening a screw with an alan wrench, it is currently loose on my bicycle (I haven't been riding around like that)
Is it possible to regain access the rest of my gears and reduce chain slipping? What is the proper technique to resolve this issue of gear shifting?