It could be anything in the circuitry that supplies current to the armature....a corroded connection, dirty switch or a wire that just let go. That's assuming it IS the armature power that's at fault but a weak output would make that near the top of my list. I included the link to the brush supplier since a bad connection can be fixed by a shadetree mechanic but brushes, well, they are "special" and hard to fake. And hard to find too.
That is one thing the cut-rate angle grinder folks got right. The bearings chatter, the plastic housings are recycled Yugo floormats but they usually give you spare brushes. A quick armature dressing with an instrument file, scraping between the segments with a dental pick and putting in new brushes and they live again.....
Harris dual-stage O/A
Lincoln AC/DC buzzbox
Miller 3035 spoolgun
Thermal Arc 185
Thermadyne Cutmaster 52
Angle grinders, vicegrips, the usual suspects
Two hands, tired body, not enough time...