You swapped the input connectors going into the amp, so you've isolated the problem to either the amp, the speaker wires or the speaker. I assume that your headphones plug into your pre-amp.

Now, swap your speaker connections (right speaker to left channel, left speaker to right channel) and power the unit up again. If the speaker hooked up to the left channel is still dead, likely it's a problem with the amplifier.

Assuming that it IS the amplifier, unplug it and pop the cover off. Check the fuses near the power supply (usually near where the power cord enters the unit). If one looks bad, pop the fuse for the working (right) channel in its place.

Let us know what happens.