hi Tobias,
sorry to hear that. I think you need to go for re-install option.
To back up your ABAP developments: There are some tool that you can download your ABAP objects and import it to other SAP system. E.g.
http://code.google.com/p/zaplink/
https://cw.sdn.sap.com/cw/groups/saplink
Cheers
m./