Here's what I would do:

These instructions are based on Self Hosted Forms Version 11.0.2311.50556.

1. Go to the Monitor page.
2. See if the Instance ID column is displayed or use the column picker to add it if it is not.
3. Search for:
   1. Process = All Processes
   2. Start date = All
   3. Started by = All
   4. With error log = All
   5. Instance Status = Select all
4. This might take a bit to load, depending on how big your system is (how much history of instances you have is).
5. Once the records have loaded, use the Search instances field to search the instance ID number.  Again, this might take a bit to load.
6. If that does come back with "No results found" then I would guess that the instance in question has been deleted.
7. If the instance is deleted, I would recommend checking the "Process Options" section of the Design page for the process in question, specifically the "Data Maintenance" section so see what is set for automatically deleting data and instance statistics for the process in question.  Is it following your Global Policies or is it overridding the global policies and set to something different.  How long is it saving it for?
8. If the instance is deleted, and that deletion isn't explanable by Data Maintenance policies - then you might want to explore whether it was deleted manually by a user.  If you have Audit Trail enabled for Forms, then "Delete Instance" is something it does track, and can be identified with the standard "Forms Deletions" report that is listed by default.

On the Data Maintenance part, here's a screenshot from one of my processes, where the global policies say to keep data for 2 years and statistics for 5 years, but the specific process overrides both for 120 days.

And here's a screenshot example of Audit Trail with the Forms Deletions report, showing an instance I deleted earlier today.