Morning Creative, and welcome to the forum!
Give us a little bit more info:
Make, model and age?
How you've been using it: what for (e.g. keying/removing old paint, wood finishing...)
Have you used dust extraction with it, or not?
Sanders and disk grinders are mirackes of modern engineering - how any brushed motor survives in the conditions they operate, always amazes me.
Which takes me to my main thought: in gritty/dusty conditions the brushes wear pretty fast. Does it spark noticably from the back of the motor? Have you noticed that getting worse recently?
Check everything Bob said, then check the brushes themselves. Look for signs of scoring, crumbling or just short length: They should be a tight fit - you should have to compress the spring a decent amount to fit them (if they're not the easy-change sort with a screw in plug). If they're at all loose, or there's less than the width of the brush left in the length, they may need replacing.
DISCONNECT FROM THE MAINS before opening up anything, obviously. On one occasion I forgot, and only got burn blisters from the electric shock, thankfully.
Check one brush at a time, and remember which way round they fit. Brushes bed-in, and should be put back in the same orientation they came out.
Also check the commutator itself (the rotating copper strips). Look for scoring, chipping and any other signs of overheating and damage. If that end of the motor is covered in crud (when you take the cover off) that's got to be cleaned You can gently clean commutators with wet+dry paper, but clean round them thoroughly first, otherwise you risk rubbing grit into them, which will make matters worse. An artist's paintbrush and vacuum cleaner will help do all the nooks and crannies.
Finally, there is a suppression capacitor fitted somewhere inside, probably near the back end of the motor. It has several functions: it stops arcing across the switch contacts when you stop the tool, it reduces electric interference (on the radio, for example). there may even be a couple of small ones, near the brushes. These sometimes fail, resulting in a lot of arcing, which in turn wears the brushes very fast and can damage the commutator. A clue is damage on the trailing edges of the copper pads (work out which way round it's turning or watch it start or stop). If you suspect that, you probably need a shop that does tool repairs.
So a few things to check there... ;-)
Hope you fix it...
E.
PS: I hadn't noticed, because of how I usually browse the forum - this section is usually for human-powered tools. It doesn't hugely matter, but the General section is probably the right place...
