I can't access the frontend of my site http://magento.p292682.webspaceconfig.de/ anymore. However, I was able to access the site from a machine that had a cookie saved from that site (10 hours earlier). Deleting cookies from that machine made the site unavailable again so I'm 100% sure that it is a cookie problem. I don't know what happened but cookies are not set anymore (I think that this is the problem) and therefore the site is not available anymore.
if you have a cookie problem issue u can System menu>Configuration>Web button in the General section on the left>Session Cookie Management panel on the right. When you expand the panel you'll see an option Cookie Lifetime. Change the number to whatever you want (it's in seconds) and click on the Save Config button