My 20" Black & Decker

The Part Schematic shows this

https://www.ereplacementparts.com/black-and-decker-tr1700-hedge-trimmer-parts-c-4167_9523_9566.html

The video may be useful in reverse order.

https://www.google.com/search?q=Black+and+Decker+TR1700+reassemble+the+blades

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Hi there isn't an internal reset it's most probably the fuse in the plug.

Check whether there is a fuse in the mains plug, this may have blown.

Here is a diagram with parts lists and online prices for the machine you stated.
http://www.ereplacementparts.com/black-and-decker-nht518-volt-cordless-dualaction-hedgehog-hedge-trimmer-parts-c-4167_9523_7848.html

Since the problem is definately a safety issue I'd call B&D and tell them you want it replaced. If you don't get a satisfactory response file a complaint with the Consumer Product Safety Commision (go to usa.gov for link info.

You are talking about a string tirmmer, right? Leave the guard where it is; it's there to protect your feet and legs from getting hit with twigs, rocks...anything the trimmer might kick up.

If you are using it as an edger you can keep the guard where it is, just flip the trimmer to do the edging.

You can order these thru black and decker at blackand decker.com

This is actually a 2.6A Hedgehog 22" B&D: I found an excellent website at toolpartsdirect.com with an exploded diagram and easy ordering of the parts required.

gearbox!! get rid of it

There is a smalll white part below the spool called a "lever". This is likely in need of replacement. Call Black and Decker at 800-544-6986 with your Model No, and Date Code (stamped on the trimmer itself near where the extension cord plugs in). If it is still under warranty they will send you the replacement part - if not, I loooked it up and it's less than $3 for the part itself.