If you have a cable-actuated clutch, that means there is a cable running from your clutch lever to the motor, instead of a hydraulic line. The most common problem cable clutches face are cable failures. These cables can stretch until they snap, or just get out of adjustment. This will cause difficult shifting or getting into neutral, or a feeling that the clutch is stuck. Also, if the cable is really gunked up, it will be difficult to pull or will not retract on its own. In this case you can take the cable off and try to lubricate it by shooting WD40 down the sheath, but if it is far gone then it will need to be replaced.
Here is how to properly adjust the cable:
You may have a problem with your clutch itself. Sometimes parts of the clutch basket can get dislodged or damaged, or you may have worn discs or clutch springs which are too compressed. This may cause slipping, grinding, or the clutch simply may not work. Here is how to investigate that: