This problem may indicate an issue with the chainsaw's power switch. If the switch is bad, then it must be replaced.
Look to see if the brake is on. If it is, release the brake and the chainsaw should run properly. If the brake is off, this may indicate a problem with the clutch that needs to be repaired.
Check the cutters on the chain to see if they are dull or broken. If so, sharpen the chain's cutters. If the cutters are in good shape, check the guide bar. If it is bent or broken, replace it.