I should have mentioned that given the error message, you should first check your web.config file and search for the "ConnectionString".  I'm not sure how this could be returning <NULL> but would imagine if the web.config flie is not well formed this might happen (I would expect the error to be different though, indicating the tag name that is causing the issue in the xml web.config file).

At any rate, take a look at this file first.  If you are not using or do not have access to Visual Studio for development your site, there is an Open Source notepad utility called Notepad++ that will provide color coding of XML files (as well as many others...).  You can find this tool at http://downloads.sourceforge.net/notepad-plus/npp.4.0.2.Installer.exe?modtime=1171587644&big_mirror=0.  If you don't see the color coding click the "Language" menu and the "x" on your keyboard for XML ... since you a file with an extension of .config rather than .xml.

Hope this helps...