know a structural engineer?
edit: nevermind, where I am, sill plates aren't usually made of wood, but if yours was wood could it have rotted away (as opposed to crumbling from structural movement)?
Some general contractors to give you free estimates. They'll tell you whether it will take a specialist or three or just them. Definitely get multiple quotes.
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.
At the corner of my house, where the wall meets the foundation, instead of the wooden board that holds up the house there is a hole that leads to the great outdoors
Depending your handiness may be able to replace it yourself, first step is to replace the hoists, then get some new wood, then maybe some new siding. Depending how much weight a car jack or two may be useful to hold it up for you.