An issue I had was the application was stopped and even if I started it, it would stop directly after trying to load the web application.

A domain policy change had impacted the service account running the application pool.

And even though my service account was a local admin on the sharepoint host, I was not able to change my local security policies (secpol.msc).

After some back and forth with the domain admin, including the service account in a security group which had "log on as a batch job" rights resolved the issue.

In short, it may be the case that a domain group policy overrides an essential permission of the application pool accounts called “Log on as a batch job”. Without this permission, the application pool account is not able to run the application pool.

This is the article that finally got me on the path to a fix: