Most Wellington calls start the same way: the door won't lift, or it lifts about six inches and stops. That's usually a fully broken spring, and if you look up at the top of the door you'll often see a visible gap in the coil.
Before the full break, there's usually warning. The door gets noticeably harder to lift by hand, the opener strains and makes a grinding or straining noise it didn't make before, or the door drops unevenly - one side lower than the other - because on a two-spring setup, one side has already gone while the other is still holding.
A loud bang from the garage, sometimes described as a gunshot, is the spring itself letting go. If that happens, don't touch the opener button. The cables and drums can still be under load even after the spring breaks, and running the opener against that load risks the cable jumping the drum, which turns a spring repair into a bigger job.