Is it happening suddenly or have you made any changes in your environment?
Open HelpDesk System Audit Log and check if you can see any records for Incident. You need to pass the EntryID of the incident as an parameter to the field Original Request ID.
Enable logs and see if data is getting pushed in the audit log form.
It happened suddenly. No changes were made that we are aware of.
Where can I find the HelpDesk System Audit Log?
You can open it from the object list or modify the mid tier URL to open this form.