How can I fix my Ryobi grass trimmer?

That happened to me. First check to see if the battery has, in fact, charged. If the battery is OK, it is probably the electric motor, which can be replaced if you take the time and effort to dis-assemble the cutting head. Since the motor costs about 1/3 of the new machine, it is worth doing.
The manufacturer advises you to advance the cutting string by banging the head on the ground. DON'T YOU BELIEVE THEM! It can drive the shaft of the motor inward and ruin it. When you need more string turn the machine upside-down, depress the centre button, and CAREFULLY pull the string out by hand. [that the machine is not operating goes without saying]

john
August 2006