We mightve made one mikes volume a little bit louder but thats it.
You seem to be resisting the idea that anything at all could be the matter. So, if you really believe that
nothing has changed and this just started suddenly, all on its own, then, the only thing it could be is
sudden equipment failure. If all the components are the same, and once upon a time, it was all working perfectly, then it has to be a change/breakdown in some piece of equipment, doesn't it?
If I were you, I'd quit looking at it as "the P.A.'s fault", you keep saying, "Nothing's changed, the P.A. just started...." You don't know that it's the P.A., then, if nothing changed, do you?
It could be some other component (as suggested throughout this thread). Why would you assume it's the P.A.'s fault?
Look at this way. Let's just say that you had a mike and it was working great. Then one day "the P.A. started squealing." One of the first things I'd check would be the mike membrane (if it has one - they work differently, most have some delicate kind of membrane for picking up sound, at least the ones we have do).
The tiniest hole or scratch in that membrane - and wow, will you ever get feedback. Where once there was X thickness of membrane keeping misc. sound out of your system, now there's x-the scratch or even no x at all if it's a hole.
Then, you get mucho feedback - but it's
not the P.A. system in that case, it's the mike.
So a cheap fix is: get two brand new mikes at a store that lets you take mikes back. Problem fixed? At least one of your mikes is bad. Swap one in, check, okay still? The other one is bad.
Etc. Same thing can happen with pickups too and other parts of the system - but you're presenting your details in what seems to be kind of a random order, so it's hard to tell what else could be causing it (we've had feedback from an acoustic pickup that got jarred and repositioned just a tiny bit - enough to make huge squealing noises, however).