It's imposssible to diagnose a problem like this over the internet.

You have a few options:
1. Buy suitable equipment (multimeter) and learn how to test the signal path through the mixer to find out which component or connection is faulty. You may have to source parts if it is not a simple soldering job.
2. Find a person in your country (or the world) who is capable to repair the mixer. Either a specialist or someone with a good head for electronics.
3. Bury the mixer with full honours. It is about 20 years old. It had a good life etc.

Repair will probably cost a bit, and a better mixer can be had for cheap enough (DJ Tech DIF-1S ?).

You can also do simple tests like check if it is just the phono pre-amp that is busted, by testing with line inputs. Anything you can do along the signal path to identify where the fault is, will be helpful (do the lights on the left light up? can you hear the left channel in headphones etc.). Open it up, check for burnt components...