Playing at rehearsal last week I suffered a catastrophic loss of volume (pretty much 99%) whilst playing. The problem was not the amp, because I plugged directly into it afterwards and it worked fine. I normally run through a chain of effects so I feel the problem must be something to do with that, I just need to find out and fix whatever it is that is going wrong…
I play a USA fender strat and have a standard 9V power unit with 5 inputs.
I use:
Digitech Bad Monkey OD
Digitech Hardwire RV-7 reverb
Marshall Tremolo
I am hoping it is just the power/patch cables cos they’re easy enough to replace… But could be the power unit itself? Do I need to invest in something better?!
Welcome any advice!
Could be one of the pedals malfunctioning - I would test each one individually and try to identify it.

I don't think an instrument/jumper cable would cause this - mainly you would get crackling or bad noise from my experiences.

It also could be, like you mentioned, a power drop from a patch cable. Same thing happened to me when I powered up my whammy pedal incorrectly.

Best bed either get something that checks voltages across the lines or test everything out individually.

I am not too savvy with this stuff so I am sure someone else will have a better idea.
-Gibson Les Paul Studio - Gem Series
-ESP Eclipse II Standard
-ESP Horizon FR II Standard
-Marshall TSL 602
-Peavey 6505+
-Mesa 2x12 Rectifer Cab
Run through each pedal individually, that will rule out the pedals. Then I'd test each patch cable, its most likely one of them.
It could be the power supply or its cables too, but thats not as likely as a cable.