Can't access my files

Which files are you trying to access (eg. System Files, Your own files), and where are they located (eg. Documents, Applications)?

If all your files are missing

You may have logged in as a different user, such as Guest. Open the Users & Groups pane in System Preferences. Your name should be at the top of the user list, under Current User. See also this support article.

If files are missing from one folder

Change the Finder view mode; for example, from icon view to list view, or vice versa.

If files are present, but seem outdated

You may have started up from a different volume (disk.) Open the Startup Disk preference pane and check the selection.