A one-minute check
With the door fully closed, pull the red release cord down and toward the opener. That disconnects the door from the opener, so you can lift it by hand.
If it lifts with little effort, about 10 to 15 pounds, the door is fine and the problem is the opener: see opener repair. If it feels like 100 pounds or more, a spring has broken: stop, leave the door closed and see spring replacement.
Only do this with the door fully closed. With a broken spring, a door released while it is open can fall.
Sources: Eric's emergency checklist: Garage Door Won't Open?; 16 CFR 1211.16, the safety instructions every opener manual must carry (eCFR)