My amp may have been left on with one speaker unplugged and is now not working. What's the likely diagnosis?

It is impedance that matters. You can connect 1000w speakers at 1 watt amplifier as long as impedance (Ohms) is correct. You can not frie speakers with low power. It is the same as turning volume down (even to zero?).If it worked for some time then problem is in mixer itself and would have happened anyway. Could be just blown fuse or something. Does it show any "sign of life" ? LEDs etc?

You haven't given us much information... You obviously have some sort of feedback and saturation in process. First thing is to unplug ANY signals into the unit including any inserts and see if problem goes away. If it does, start adding them back in. If it doesn't go away with only the main outputs then VERIFY that you are using BALANCED interconnects to your amps/speakers AND that the mixer AND all of these amps/speakers are powered from the SAME receptacle or power conditioner module. If the BARE mixer with only headphones has the problem, then you have a power supply in the mixer that is squeging.

In spite of your description it is not quite clear of the connections... In general you should limit the number of speakers connected to an amp output to a maximum of two. I am not sure if you might have more than that in your daisy chain. Let's go over another important thing: are ALL of the amps and the mixer (desk?) powered from the SAME receptacle or power conditioning module? If not, make it so even if you have to run extension cords. The ground bounce of building power at venues can destroy your equipment. The brand/model of the amp(s) and the mixer would be useful to analyzing the problem. It is unlikely the failing battery in a guitar would kill one side of the PA. If the amp is overloaded, the thermal cutout may be disabling part of the amp and the system. Also please verify the RMS capability of your amp. You want the power of the amp to NOT exceed the rating of the speakers USE RMS VALUES... not the fakey "program" or "peak" ratings... those are just advertising gimmiks to show my numbers are bigger than yours... we know that game...

Something similar happened to me a while back,

Check your fuses, some amps have 2 fuses on the back and it sounds like your second one might be blown.

If its not that, then it is probably the amp module and you will probably have to take it for repairs. Be careful if you open it up yourself, sometimes the amps can hold residual charge and there is a danger of getting zapped.

Hope this helps!

Have you connected the Left speaker to the right speaker using the supplied cable? If not, connect the cable to the socket on the back of the right speaker labelled "to left speaker" and on the left speaker, to the socket labelled "from right speaker"
If this still doesn't solve the problem, try swapping the input cables (put the white plug in the red socket and vice versa) to see if this makes the left speaker produce sound, if it does, check the connection at the source to check the plug(s) are in all the way. If there is still no sound in the left speaker, unplug everything and do the following:
connect the left and right speakers with the supplied phono lead (Probably black) between sockets labelled "to left speaker" and "from right speaker"
then connect the source to any of the inputs on the back of the right speaker (ensure if using the red and white phono inputs that both are connected)
finally plug in power source.
Hope this helps

This can be speakers, cables, connectors or the finals in your amp. First is just switch the speakers. That rules out the speaker etc. Just swap the leads from right to left etc. So the working setup is on the suspect bad output. If it works then the problem is something towards the speaker. If not its the amp. You can then try switching just cables to rule those out. Should take just a few minutes to figure out what the problem is. If it is the amp end check for dumb stuff like switches or the volume ;-] If not its a trip to your repair shop. Cables and contacts can be easily damaged and are the first suspect. Speakers are usually bullet proof unless you do everything maxed out. Amps will have issues due to age or heating etc. Just make sure to check out the amp value before telling anyone to fix it. You don;t want a big repair bill to fix a inexpensive amp, just replace it.

If you connected an external speaker plug from one into the same type plug on another amp, you have driven signals backward into the amp that was off... NOT into the speaker in that unit.

The usual result is blown parts in the amp that is off.

Some amps have their own speaker cable that you can unplug to connect to another amp and that is fairly safe. NEVER run an amp without a speaker load on it.

It sounds like the speaker output jack is broken or it has come unsoldered from the circuit board. You can fix this if you have enough electronics experience but you have to be very careful as components in an amp can carry a lethal charge even after being unplugged. You can partially discharge the capacitors by unplugging your amp from the power source with the speaker plugged into the working jack and set at a low volume while turned on ( not on standby ). If you feel at all uncomfortable attempting the repair yourself you are far better off taking it to a repair shop preferably one with experience with Marshalls. The same is true for other brands as well. Please let me know if this helps.

It sounds to me like you have blown the internal speaker.