Nearly every "it has disappeared" turns out to be one of four things. Check them in this order.
This topic includes:
1. The wrong project
Look at the project name at the top left. Everything except your own profile is scoped to one project, and the selection is remembered between sessions — so the project you are in is the one you were last in, not necessarily the one you are thinking about.
2. A filter left on
Registers remember their filters as you move around. A status filter set last week will still be set today, and an empty list is what a filter that matches nothing looks like. Use Clear filters.
The same applies to the specification set and division filters, the album filter in Photos, and the version filter in Schedule.
3. A permission
If a tool is missing from the sidebar rather than empty, your role does not grant access to it. Roles are set per project, so having access on one job says nothing about another.
Ask a project administrator to check your role — see Roles and permissions.
4. It was deleted
Check the tool's recycle bin. Deleted issues, files and photos go there and can be restored, along with a record of who deleted them and when. The activity log will say what happened even when the bin has been purged.