Hey there, Christian.
So I assume you want to keep the data?
Basically I would suggest that you try the card with a different device, to see if the same thing happens. If you are still unable to access it and backup your data, you could give it a try via Ubuntu Live USB/DVD (this is basically a freeware, bootable, portable version of Linux), to see if it recognizes the partition properly and if you can get to your files. Another option would be to give it a go with data recovery software:
http://www.tomshardware.co.uk/forum/id-1644496/lost-data-recovery.html & http://pcsupport.about.com/od/filerecovery/tp/free-file-recovery-programs.htm.
Hope that helps. Please let me know how it goes.
Boogieman_WD