Like Kepy mentioned above, it probably the switch. Dust gets into it and gums up the contacts. As a scroll saw instructor at the local Woodcraft store, I have repaired many a DeWALT switch. If you are handy it’s doable. The first thing you want to do is remove the cover that holds the speed control and switch. Unplug the machine. With needle nose pliers remove the two wires that connect to the switch. Again, make sure the machine is unplugged. Tape together the two wires, then plug the machine back in. If it comes on, the problem is in the switch. If not, then it’s time to take it in to the repair shop.
The switch is is repairable. If you need to, I’ll walk you through it. Let me know.