Mountain Biking - Rear Gear Doesn't Work

my shimano sis gear at the rear doesn't seems to work. everytime i switch to lower gear it produces some cranking noise. anyone knows what to do?

Hard to say, given the description. More information would help. A few possibilities

Your shifter is broken. In this case the cable is not being pulled tight enough to change the gear.

Your cable is broken / frozen. In this case, pressure at the shifter is not translated into tension at the rear deraillieur.

Your rear derailleur is shot / frozen. In this case, tension on the shift cable does not move the derailleur to the left (as seen from the back of the bike).

http://www.parktool.com/repair_help/index.shtml
Sounds like a limit screw adjustment to me.

Actually if its grinding when you shift, the cable has most likely stretched and doesn't have enough tension to push it fully into the next gear, so re-tension the cable.

Actually if its grinding when you shift, the cable has most likely stretched and doesn't have enough tension to push it fully into the next gear, so re-tension the cable.

Sorry I got that in reverse. Sometimes I immediately think lower as being down the cogset.

Thanks guys. really appreciate your time to reply and the valuable info. now i know what to check.