Angle grinder head is worn how can i fix it - Black & Decker Black Decker Decker G750 4 12" Angle Grinder 6 Amp

The motor brushes could be bound up in their holders or worn out. Another possibility is the power switch is worn out or packed with dust not letting the contacts in the switch come together.

Buy the new brushes from the manufacturer. You already have the old ones out - put the new ones in the same way you took the old ones out.

it would be much better if you bought a new one because it would be cheaper

It is time to replace the trigger switch.

Inside the head of your grinder there are a couple beveled gears, one on the armature and the other on the spindle that the discs attach to. If either or both gears are broke or worn down to where they don't contact each other properly, the motor will spin but not turn the spindle. It's easy to disassemble the head to check but if the tool is that new you may want to look into getting it repaired under the warranty.

Obviously too late for you, but it may help others.
I had the same problem with a new Makita 18v lithium grinder and this worked.

Remove battery, remove brushes. Using good quality cotton buds, dip in Methalated spirit and insert into hole for brush down to armature, slowly rotate head of machine. Change cotton bud and repeat process, then clean the brush heads. Re-assemble and try. Note: Meths will dry completely leaving no residue, unlike turps etc.

I would just buy a new one.
parts could cost almost as much.
don't forget the rest of the grinder is just as worn out too

It either has worn out brushes or switch fault, take it to a power tool specialist for repair.

brushes or bearings worn out

most likely the carbon brush is worn out and need to be replaced.