Electrical – Basement leak behind electrical panel

What this could be is that you have a slow leak from the floor above. Probably a water feed rather than a drain. Although it could be from the heating system if you have a water or steam system with radiators. When you turned the water on the house it started to drip, and the ceiling soaked it all up. Eventually the ceiling became saturated and started to drip onto the floor.

If you do have a slow leak then obviously you need to find the source. It's probably a leaky pipe joint, or maybe a loose compression fitting. The only way you're going to find it is to get access, as for how that completely depends on your house's construction. There may be a panel in front of the bath that can be removed, etc.

What you need to do is meticulously go through the whole bathroom above and look for any kind of access to pipes you see, and look for signs of dripping moisture, oxidation on the pipes, or hard water deposits. As it sounds like the whole ceiling needs to come down on the floor below do that first, then use that access to find your leak.

As for how to fix it if it's a compression fitting get out your wrench and have at it. If it's a solder joint or a bad pipe get a plumber in unless you have the tools and the experience. Good luck!

Water will always win if you "fix" these from the inside.

The correct, effective, and unfortunately expensive fix is to excavate outside and drain the water below wall level outside, then seal and drain the exterior of the wall to the exterior drain.

You may choose to chisel out this mess and try fixing from the inside again, but it will almost certainly fail again. The other inside approach is to "capture" the leak and lead it to a sump.

If tilting at the windmill, this image from "www.oldhouseweb.com" illustrates the correct shape for chiseling or diamond-blade cutting out the crack before attempting to re-fill it. At the bottom of their page they note that their information came from Michagan State University Extension publication E2109, and the image would appear to also have come from that publication originally.