If your chef-server is not starting on 12.04
There is a relevant bug at : http://tickets.opscode.com/browse/CHEF-2766
This most likely happens if you didn’t run “apt-get update” before you installed chef-server

try running chef-server on it’s own and see what messages you get.
$ chef-server

Loading init file from /usr/share/chef-server-api/config/init.rb
[Mon, 25 Jun 2012 06:31:53 +0000] INFO: Loading roles
Loading /usr/share/chef-server-api/config/environments/development.rb
[Mon, 25 Jun 2012 06:31:53 +0000] INFO:
[Mon, 25 Jun 2012 06:31:53 +0000] INFO: ********************************************************************************
[Mon, 25 Jun 2012 06:31:53 +0000] INFO: *   Starting Chef Server in Development Mode.
[Mon, 25 Jun 2012 06:31:53 +0000] INFO: *   Start the server with `-e production` for normal use
[Mon, 25 Jun 2012 06:31:53 +0000] INFO: ********************************************************************************
[Mon, 25 Jun 2012 06:31:53 +0000] INFO:
NOTE: Gem.activate is deprecated, use Specification#activate. It will be removed on or after 2011-10-01.
Gem.activate called from /usr/lib/ruby/1.8/merb-core/core_ext/kernel.rb:142.
/usr/lib/ruby/vendor_ruby/1.8/rubygems.rb:232:in `_deprecated_activate': Deprecated use of Gem.activate(dep) (ArgumentError)
        from /usr/lib/ruby/vendor_ruby/1.8/rubygems/deprecate.rb:63:in `send'
        from /usr/lib/ruby/vendor_ruby/1.8/rubygems/deprecate.rb:63:in `activate'
        from /usr/lib/ruby/1.8/merb-core/core_ext/kernel.rb:142:in `load_dependency'
        from /usr/lib/ruby/1.8/merb-core/bootloader.rb:405:in `load_dependencies'
        from /usr/lib/ruby/1.8/merb-core/bootloader.rb:405:in `each'
        from /usr/lib/ruby/1.8/merb-core/bootloader.rb:405:in `load_dependencies'
        from /usr/lib/ruby/1.8/merb-core/bootloader.rb:393:in `run'
        from /usr/lib/ruby/1.8/merb-core/bootloader.rb:99:in `run'
        from /usr/lib/ruby/1.8/merb-core/server.rb:172:in `bootup'
        from /usr/lib/ruby/1.8/merb-core/server.rb:42:in `start'
        from /usr/lib/ruby/1.8/merb-core.rb:170:in `start'
        from /usr/sbin/chef-server:86

Fix the offending line:
$ sed -ie ‘142s/Gem.activate(dep)/dep.to_spec.activate/’ /usr/lib/ruby/1.8/merb-core/core_ext/kernel.rb
$ /etc/init.d/chef-server start
$ /etc/init.d/chef-server-webui start