If the fuel has been in the tank unused for months, it can pick up moisture and fail to ignite (that's why you should drain the fuel before storing the chainsaw or add fuel stabilizer to the tank). Replace the fuel with fresh gas if it's old and untreated with stabilizer. You might also need to replace the fuel filter at the same time.
Performing preventive maintenance including a tune-up will also help the engine run smoothly. Check the fuel lines for cracks and replace them if necessary. If you still have problems after checking those issues, consider replacing or rebuilding the carburetor.
