Did you try swapping out the cable to the speaker?

Did you try swapping over the speakers?...

if the speaker still works when you swap it over...chances are its the interface.

Cabling is always the first thing to check (power and audio).

Do it have a pilot light? Is there power? Has the monitor's power cable come loose at either end? Is it plugged into a cheap generic powerstrip, or good rack mount power conditioner? Do the monitors have any external fuse or breaker?

Definitely swap left/right audio cables as Sean G suggests. Observe any changes in behavior. It will be useful information to know whether the one working speaker still works plugged into the other output channel, and vice versa.

If that doesn't get any results, then with the speakers turned off, and no music playing, you might try turning the volume knob on the interface from zero to 10 and back a dozen times. Do the same thing with the volume controls on the speakers. Then power things up and see if it behaves any differently. If a potentiometer sits in the same place all the time, it can build up oxidation or other crud on the path. Running the knob through its full range of motion (with nothing playing) may knock that crud loose and restore or improve contact.

It's unlikely it's a raw speaker. Under normal circumstances, the woofer and tweeter aren't likely to both die at the same time unless there was catastrophic failure. (the kind that usually involves a small cloud of acrid smoke)

Depending on your results, it might point to the interface's volume knob/potentiometer, the output connectors on the interface, the cables, a fuse or breaker in the monitor, the volume pot on the monitor, or the input section of the monitor.

Let us know what you find.