Bump knob is loose but still wiill not come off?

I would recommend using something a bit more hefty like self adjusting locking pliers or even a plumbers' pipe wrench. If you can get the bump feed to free up, but it's out of trimmer line you may be able to manual spool it. Found a how-to vid on youtube that may be relevant to your model Ryobi trimmer -
Howto string Ryobi trimmer

You need a new bump knob because it is worn out beyond it's useable capacity.

Sounds like a faulty blend door actuator. This part diverts air from the different ports ie floor, vents, defrost. If it keeps changing when you hit a bump, either the actuator has a loose connection or it's bad all together.

On my petrol strimmer you have to "bump" the real while it is spinning. It may be the same with the electrical version.

A typical trimmer has a small bump knob, an inner spool with the plastic string wound on it and the housing for that which is the string head.To remove the small bump knob, hold string head firmly and turn the bump knob counterclockwise. To remove the head entirely, first remove the bump knob. Next, remove the inner spool with string. What should be left is the spring with the end of the gear shaft showing. Some heads are locked on and some just slip over the end of the shaft. Check the gear drive just above the string head for any holes in the collar. An object can be placed in one of the holes to lock the drive and twist the head off.Ryobi's use retaining clips on the shaft which have to be removed.

You should be able to remove the bump knob and then the inner spool. To keep the shaft from turning, try to grasp the dust cover with some vise grips or large adjustable pliars to hold it and just remove the knob. Spray penetrating oil under it if necessary and let it soak a couple hours to remove rust and corrosion and try it again. If there are any holes above the head in the dust cover or arbor, try to insert something in there to lock it up, like a slender scredriver. Here are the line spool removal instructions. See pages 14 and 15 in this manual.
http://dl.owneriq.net/e/e795beb6-5dd6-4dbd-a82f-57b8d1a69355.pdf
You will need Adobe Reader installed on your computer to view. It is a free download if you need it.

One of your power wires is loose. There should be two of them. One is constantly connected to +12 volts and one is connected to the accessory on your ignition system. If your clock resets after you hit the bump, it is your constant power that is loose. If your clock stays with the correct time, your accessory wire is loose.

Align the arrows under the bump feed, push in on bump knob, unscrew in counter clockwise direction, remove spool. Wind string in direction arrows advise on the spool, reassemble. You will be back to weedwacking asap.

Unscrew the bump-knob and the rest pulls off. If it won't unscrew you could always cut it off and buy a new one from ALM Manufacturing 01423 797208