Look for loose wiring connector, otherwise:
Temporarily swap left and right speakers.
Plug in and play a continuous recorded tone of about 50Hz-440Hz.
Play with maximum volume to get a decent voltage reading.
If LEFT CH now works then L speaker is faulty.
If RIGHT CH now works then L amplifier is faulty.
Electrical test (needs a meter with Volts and Resistance).
With speaker still connected test audio (AC V) signal on working right speaker terminals.
Now check left Channel at amplifier board if possible, and at speaker terminals.
If no V unplug speaker wires if possible at board then test across board L+ and L- output pins.
Still nothing means amp is faulty.
Test continuity of speaker wires end-to-end. Each wire should be 0 ohms or close to zero. The continuity buzzer function can also be used for this.
With amplifier off and unplugged, remove speaker-connector from board or cut live speaker wire or unplug lug at speaker, whichever is easier. With one wire apart from the amplifier, test the loudspeaker ohms.
Should be in the range of about 4-8 ohms.
Most speakers have the rating screen-printed on the voice-coil magnet housing.
If ohms reading is way too low, say 0-1 ohm it has a short and is damaged.
If ohms reading is way too high, say several Meg-ohms, it has a coil break and is damaged.
Carefully join/tape any cut wires if applicable, renew speaker if faulty with same size and rating, re-test then re-assemble.