I started a new job recently where I am working on a very large application (15M loc). In my previous job we had a similarly large application but (for better or for worse) we used OSGi, which meant the application was broken down into lots of microservices that could be independently changed, compiled, and deployed. The new application is just one large code base, with maybe a couple of .dlls.
So I need to change the interface of this class, because that's what my boss asked me to do. They initially wrote it with some assumptions that didn't generalize too well, and for a while they have been avoiding the problem of refactoring because it's so tightly coupled. I changed the interface and now there are over 25000 errors. Some of the errors are in classes with important sounding names like "XYZPriceCalculator" which reaaally should not break. But I can't start up the application to check if it's working until all the errors are resolved. And many of the unit tests either directly reference that interface, or are coupled to base classes which reference that interface, so just fixing those is a pretty huge task in itself. Plus, I don't really know how all these pieces fit together, so even if I could get it to start, I don't really know what it would look like if things were broken.
I never really faced a problem like this at my last job. What do I do?