You get this error:

Service Unavailable

HTTP Error 503. The service is unavailable.

The application pools were failing to start as they were not able to login using the proper account. Here is the error:

Application pool plesk(default)(2.0)(pool) has been disabled. Windows Process Activation Service (WAS) did not create a worker process to serve the application pool because the application pool identity is invalid.

Reset the password on the two accounts:

IWAM_plesk(default)
IWPD_11(unique)

Then reset the login creditendials for the two application pools. The application pools started and the sites are working again.