According to the man page, you should be able to revert to the latest lucid package with apt-get install 'apache2=2.2.14-5ubuntu8.9'. You may also need to include other packages on the command line such as 'apache2-mpm-prefork=2.2.14-5ubuntu8.9' 'apache2-mpm-worker=2.2.14-5ubuntu8.9' 'apache2.2-common=2.2.14-5ubuntu8.9'. If your experiment caused other packages to be pulled from the maverick repository, you'll probably have to revert them as well.