If you have tried the tests mentioned above and nothing seems obviously wrong with the motor I would suggest the on off switch, dust creeps into the housing and prevents the contacts touching, try a blast or two of high pressure air to see if that can get in to clear the contacts, if not first see if you can get into the switch to clean it up if not as it is off the router get it tested for continuity. If it fails a new switch is in your future.
If I buy a power tool with an on off switch that sits in with all the dust it creates I try to get it out, maybe on a wooden panel fastened to the outside, table saws are particularly bad for this. Doing this years ago stopped a recurring problem I had on my table saw.
I hope this helps.