Duane's reference to the DNN Blog indicates that DNN 05.04.03 has been pulled from download distribution today. While I do recommend that you restore your site to 05.04.02 using your filesystem and database backups and await the release of DNN 05.04.04 most likely later this week, I do not believe that the ExecuteSQL error described in the Blog and with which I am very familiar is the reason for your difficulties. Before 05.04.03 being pulled today I had already upgraded several development sites and two production sites to 05.04.03 without any issues in triggering the upgrade or in logging in after the upgrade.
So that we can better diagnose what is causing your upgrade to fail (and before your roll back to 05.04.02), please answer the following:
1. Which type of 05.04.03 package did you use - Install or Upgrade package?
2. Did you make any manual changes to your site's web.config file before the upgrade attempt?
3. From the web.config file what are the value of the following keys in the <appSettings> section?
a. AutoUpgrade
b. UseInstallWizard
4. If you have access to your site's database using SQL Server Management Studio or similar tool, what are the following table column values:
a. Major, Minor, and Build in last row of Version table?
5. What is the file version of the assembly DotNetNuke.dll located in the site's \bin folder? If this is a local installation hover over the file then select properties. If it is a remote install, download the assembly file via FTP, etc. then check version in the same manner.