The symptom points at the component
When a door stops working the behaviour is usually specific, and specific behaviour maps to a specific part. This guide connects the common ones so you can ask for the right service rather than a general call-out.
Two rules make the rest of it work. First, pull the manual release and move the door by hand: if the problem persists it is the door, and if it disappears it is the opener. Second, if the door will not hold its own weight at mid travel, the counterbalance is the issue whatever else is happening.
None of this replaces a look at the door. It does mean you can describe the fault accurately, which is worth more than it sounds when someone is deciding what to put on the truck.