Well it is not proven one way or the other yet if you were hacked.
I do not believe openid is enabled by default and that shouldn't prevent the login screen from displaying. I also do not think there are any issues with Joomla and php 5.3.10.Now, I think your site has been hacked just based upon what you have written. This is why:
1.) your Joomla version is way out of date. Current version is 1.5.26
2.) your issues appeared around the time frame multiple hosts and thousands of php sites were hit with a hack.
3.) Xmap is on the VEL list and since your site is out of date, it (and maybe other extensions) are probably also out of date and vulnerable to hacking. http://docs.joomla.org/Vulnerable_Extensions_List#xmapYou need to follow what is provided below in order to properly recover your site.
When fixing your site, use the 1.5.26 version of Joomla. Do Not use the 2.5.xx version as it is not compatible with what you currently have. So don't create further issues to fix. Security checklist 7 also contains the link to the doc page for restoring your super-admin user and password, though this is not your real issue at this point.
[ ] Run the Forum Post Assistant / FPA
Instructions available here
and are also included in the download package.
[ ] Ensure you have the latest version of Joomla
. Delete all files in your Joomla installation, saving a copy of the configuration.php file.
Replace the deleted files with fresh copies of a current full version of Joomla (minus the installation directory), and fresh copies of extensions and templates used. Upload the copy of your configuration file. Only by replacing all files in the installation (including extensions and templates) can you be sure to remove the backdoors inserted and hidden in files and directories
More detail can be found in the security Checklist 7 link below.
[ ] Review Vulnerable Extensions List
[ ] Review and action Security Checklist 7
to make sure you've gone through all of the steps.
[ ] Scan all machines with FTP, Joomla super admin, and Joomla admin access for malware, virus, trojans, spyware, etc.
[ ] Change all passwords and if possible user names for the website host control panel and your Joomla site.
[ ] Use proper permissions on files and directories. They should never be 777
, but ideal is 644 and 755
[ ] Check your htaccess for for any odd code (i.e. code which is not in the standard htaccess supplied as part of the Joomla installation).
[ ] Check the crontab or Task Scheduler for unexpected jobs/tasks.
[ ] Ensure you do not have anonymous ftp enabledNote: The forum post tool will work with 1.0.x, J1.6.x, J1.7.x, 2.5.x versions of Joomla.