Hi

It sounds to me as if either one of your speakers (or the connections to it) has failed OR that the output section has failed for the affected side of the stereo power amp.

You can use a solid state amp with no speakers attached at all or with the correct speakers attached.  It is not recommended by Line 6 to use only one speaker with any of their stereo amplifiers as various imbalances can occur in the amplifier which can cause the premature failure of one of the two power amp sections.  Read this FAQ from Line 6 which will give you their take on just using one speaker - and whilst it doesn't specifically mention the Spider IV 120, the meaning is clear - i.e. DON'T do it   http://line6.com/support/docs/DOC-4410

I would recommend that you take your amp in to an Authorised Service Centre to be checked out rather than use it and potentially making the effects of fault worse.  If the amp is still in the 12 month warranty period and you haven't mis-used the amp, there's an excellent chance it will be repaired or replaced FOC

However if your amp is not a Spider IV (I've assumed it is because this is the Spider IV forum ) and is a legacy model. then it's likely not to be repairable.

You could try swapping the speaker connections form right to left and left to right.  Make a note of which speaker isn't working before you do this!  And make a note of which wires are connected to which terminals before you do anything so you can reconnect everything in the same way as it was originally - take a digital photograph!  If the opposite speaker doesn't work when you've swapped the connections, it would seem like the power amp section for that half of the stereo amp has failed, but if the same speaker doesn't work, then it is more likely that the speaker itself has died and needs replacing and this is probably something you could do for yourself provided you buy a speaker which has the correct impedance (almost certainly 8 ohms) and (assuning the amp is a 120w model) can handle at least 60 watts of power (I'd buy a higher rated speaker of at least 80w capacity if possible - contact Line 6 Spares as they may have spare speakers they could sell you.

Nick