You might have a corrupted repository . I have had a couple customers that had corrupted Object Repository and here is how we have resolved the issue.
1. Open 'Object Repository Manager'
2. Create new repository. File > New
3. Save the new repository. File > Save
4. Open the 'Object Repository Merge Tool'. Tools > 'Object Repository Merge Tool'
5. Enter the corrupted repository in the Primary File location.
6. Enter the new blank repository in the Secondary file location.
7. Press the 'OK' button.
8. A warning message might appear stating the test object cannot be loaded.
9. The merge tool will display status of the merge and show only the items that the tool can merger.
10. Save the merge results. File > Save and make the name the same as the original repository.
11. Replace the corrupted repository with the new one created in last step.
12. Re insert missing object back in to new repository.
I am sorry this not the news you were looking but, does this explain how to get the repository back and working?