Start by not assuming the worst. One dead channel in a headphone is a chain, and the driver (the part that is expensive) is the last link in that chain. Work it in order.
1. Rule out the source. Play something through different outputs: another jack on the same device, then another device entirely. If both ears come back on a different source, your headphones are fine and the fault is in the source or its jack.
2. The cable, especially at the ends. On headphones with detachable cables, swap the cable first. On fixed-cable headphones, play something and gently wiggle the cable near the plug and again where it enters the earcup. Sound that cuts in and out as you move the cable is a broken conductor at a strain relief, which is the single most common one-side failure there is. That is a cable repair, not a headphone funeral.
3. The driver. If the cable proves out and one side is still silent, the driver itself has failed. This is a real failure class in the headphone world; makers document it as a standard case ("one channel dead"). One thing to know before shopping for parts: dynamic driver replacements are sometimes available, but planar magnetic and custom in-ear makers generally treat driver work as a pair-level rebuild done by the maker, not a home part swap. For those, the maker's own service channel is the path.
What we would not recommend: opening a sealed driver assembly to poke at it. There is nothing in there a hobby repair improves, and the shell usually does not survive the opening.
Post what you have (model, detachable or fixed cable, whether the wiggle test did anything) and someone can tell you which link in your chain failed.