Theres a multitude of potential steps we could do , or may need to do here.
But Its probably best if we start by resetting indesigns preferences, heres how.
When prompted to delete InDesign preference files, click Yes. InDesign launches with defaults and regenerated preference files.
If you aren't prompted to delete the preference files, you can delete your preferences manually.
Once the preferences are reset, try the document again.. Does the index all you to edit or add to it now? Does it show normally?
If not, then your best bet may be to check time machine and see if you have a backup copy of the file inside time machine that was made before the corruption to the index happened.
If not, consider resaving the document with a new name, and then testing the new document.
Consider copying everything from the current document, into a new document, then saving the new document and seeing if you can add to or edit the index.
I will stop here for now, and allow you some time to read, and test these potential solutions. Whether this works or not, or if you need any help with any of these steps specifically please dont hesitate to respond.
Senior Technician and Network Switching Systems Operator / Maintainer.