Get some screw style foundation support jacks, start taking off your siding to see how far the rot goes. Duct tape up some trash bags to temporarily block the elements. Depending on the damage and your level of comfort/knowledge, you can probably do a decent amount of demo yourself. Call multiple general contractors to assess, get a quote from each. You don't need an engineer, the house was already designed and engineered. A rotted out hole is as simple as finding how far the damage goes and replacing materials, then doing a better job of flashing and sealing.

It sucks and it's going to cost a lot, but better to find it now than to have a partial collapse.