You have not given us many clues to go on!

Have you had to reinstall Windows or move to a new PC and reinstall FH?

If you did not keep backups of your customised Diagrams and Icons then they will have been destroyed.

Project backups do not save customised Diagrams or Icons.

There is advice in the FHUG Knowledge Base on Using Flags and Icons and Expressions:

https://www.fhug.org.uk/wiki/doku.php?id=how_to:using_flags_and_icons_and_expressions

Perhaps your Flags and Icons were associated with using Ancestral Sources.

Do you have Census record Flags on your Individual records?

The necessary settings are in the Diagram > Options > Boxes tab in the Conditions.

But the type of Condition depends on whether you used a Record Flag per Census Year or an Expression that tested the existence of Census Events.

Do those Conditions exist in your Diagrams?

If they do exist, then only the Icon files have gone missing.

Can you remember where they were saved and where you obtained them?

If not then download some new ones from the FHUG.

Regards, Mike Tate