I would look at a couple of things for this.
1. Where is your Output File Repository located? Is the disk full?
2. Look for the Output File Repository Server in the CMC. Is it pointing to the correct folder?
3. What userID is your BO system running under (each of the individual services in XI through R2, the SIA with 3.x and above)? The default is the Local Services account which does not have access to network resources. So, if your file repository is on a different server than where BO is running, it won't have access to it. If it's using a network userID, has the password changed?
4. How much space is available on the C drive on the server where BO is running? Crystal does a lot of swapping to disk and is notoriously bad about cleaning up after itself when things fail. Normally you can find the temp files for Crystal under the path: C:\Documents and Settings\<user id>\Local Settings\Temp (this is a hidden folder!). You can delete anything in there that starts with "~" or a file extension of ".tmp" that has a file date earlier than the current date. This needs to be cleaned out periodically, but it's something they don't mention in the Admin Guide.
If there's still an issue, try stopping the Output File Repository Server and restarting it.
-Dell