Linkage may be too loose, disconnected, or broken. Typically, to get to the drive mechanism(s), you need a Service Manual. Depending on how old the unit is, and how much it will cost to repair, you would want to weigh that against the cost of a new unit. Echo, which to me is a good unit, SRM 2100 (and the like) is a decent, all-around home unit. It runs for under $200. I've had mine for 10 years and all I do is change the plug and air filter (and recycle the spent gas/oil mix) each year.
Maybe the MFGR's WEB site has a User Manual or Service Manual available to tell you how to get 'inside'. My best to ya and hope this helps.