Ivy is usually very tough.

First place to check is the potting mix. If someone put it into peat and it's dried out then it can be very difficult to rehydrate.

Being a reckless soul, I'd plunge the whole lot into a big container of rainwater, or tap water that's stood for a while. If it floats, then water take-up is part of the problem. if it slowly sinks, giving off endless streams of bubbles - ditto.

If you have access to good garden soil you could mix one part garden soil with two parts of non-peat potting mix and a half part of sharp clean grit (NOT sand). The grit lets more air into the mix and helps the water spread out.

No water crystals, and no extra fertiliser until the plant starts to recover.

If your weather isn't too variable, put it outside in a sheltered place and bring it in at night if there is frost danger. About a half day of sun or a little less. Wind shelter until it settles down in the pot.

If you find wildlife in the pot has eaten a lot of the root system you'll have to (sob) take back some of the leaves and branching. They have to be in balance so there are enough roots to send water up to the leaves. Not there - wilt and drop.

Easy on the water. When you repot give it a thorough soaking, then let it dry out until the mix is dry to about an inch down. Then water again.

When new leaves start to peek out, liquid feed. MiracleGro is pretty good, at about half normal strength. Only go to full strength after about six weeks and several new leaves fully out.

There's also the point about the annual change of leaves from the winter set to the summer producers. I'm not sure when that happens in zone 5. Here, it's toward the end of summer, but ivy rarely gets powdery mildew - hence my suggestions about the water status. Spraying powdery mildew with just water can help to control it on some species. Worth a try, perhaps?