Determining why your weed eater won't start could be a hard task especially if the problem is way too complicated. However, the common culprit of it would be its fuel and fuel line. You have to first check if you still have enough fuel. If you are using an electric string trimmer, check your power line. If using a battery-operated one, check if your battery has enough charge to operate. For further details about this, go to http://www.ask.com/wiki/String_trimmer.