Sim-kr5-230b?

Check the brushes.you might be able to get replacements from Solent Tools.
Check each wire in the cable. It might have a failed wire., Especially where it bends when entering to tool or the plug.
Check the switch
Spares are likely to be few and far between, a recurring problem with cheap tools badges up with store labels and sold cheap.
Probably better to bin it and buy new with a warranty.
Oh, one useful note: Toolstation are doing tool-less angle grinder disk nuts for a fiver each. Just got one and was most impressed.
https://www.toolstation.com/milwaukee-fixtec-angle-grinder-nut/p58972

Djdj
January 2023