You can go to File menu “Windows”- scroll down select “Layers”- it might be hidden or collapsed. You an also access Layers through the hotkey “F7”. You can select all layers by pressing F7 (brings up Layers Panel)- click once on Layers Panel, then keys “Crtl” +Alt +A- which toggles between Show All and Hide All Layers. If you cannot find your Layers Panel after toggling F7 repeatedly- try Resetting your Workspace- from toolbar menu Window-> Workspace-> Reset. Once3 you have the Layers panel visible and restored you can toggle expand/collapse all layers with Crtl +Left-click on the tiny arrow on the left of the individual layer thumbnail. Source: Accidentally ruined my workspace and layers a thousand times, panicked and finally worked out what I was doing.
