Remove screws to the inspection cover with a #20 torx driver. use a screw driver to gently pry and remover inspection cover. with a flashlight check the brushes. If the tiny spring loaded button had lifted the brush up off the commutator the brushes need replaced. using a dental pick or small tweezers lift the tension spring up and out of the way to the side opposite the brush contact wire. with needle nose pliers gently pull the clip holding the brush to the brush holder. keep your thumb over the brush holder to prevent the brush holder from too much pressure or it will break. Pull the brush and wire from the holder and install new brush. First using needle nose pliers gently attach the wire to the tab on the brush holder until it is all the way on. Then slide the brush into the holder with the wire facing the electrical tab and don't forget the tension spring. The entire back on the grinder must be removed to service the other side. It is not necessary to remove the power cord to do this. it can hang to the side. When reassembling the back to the grinder be sure the switch slider is installed properly and functions correctly before you completely assemble the rest of the cover or the grinder will not turn on and off properly. I like to carefully test the grinder before completely assembling the cover by switching the grinder on, hold it at the neck and plug it in to be sure it works.
Here is a link to Metabo's web site link removed Metabo Services-Support. Click on spare parts list, then search by category,then machines,select option small angle grinders. Find your model and type number, click on it. The wiring diagram is the small box to the right near the bottom.