get error message ""Sorry, but due to a high volume of traffic, we are unable.."

We have a WPMU site with 24 blogs that have been running for almost 3 years.

We tried to upgrade the version of WPMU, but found that we had to upgrade our version of PHP.

When we tried to upgrade PHP, we found that the Web host’s Plesk software was no longer compatible with the PHP.

So, we rolled back to the earlier version of PHP.

Now, whenever any bloggers enter their username and password on the login page, they get the error message “”Sorry, but due to a high volume of traffic, we are unable to display this web page right now. We expect things to be better soon, so please try checking back later.”

This happens on each of the 24 blogs and across several different types of browsers.

? Have any of you experienced this error message before, and what did you do to get past it?

Thank you!