In Windows 10, there is a new and improved feature to allow the user to seamless switch from Tablet mode and Desktop mode. This is to support the bridging and gap between the new Start menu and the, now old, Start menu from Windows 8.1. Windows 10 suppose to know and be able to detect when your machine is in Tablet mode and when it is in Desktop mode. To check which mode you are on you can open the notification tray and check the status from the bottom of the screen.
Sometimes Windows get confused between when to enable/disable Tablet mode
Perhaps, when Windows 10 is fully released issues like this will no longer occur. But personally, Ive encountered this a number of times, where for no obvious reasons it would switch to Tablet mode on a dedicated Desktop machine. At first, I wasnt sure what to do, since the Start menu is the only thing showing on the entire screen. Unable to access my Desktop and all the Taskbars icon are missing. It puts me into panic mode where I thought my Windows 10 is frozen in a weird state, without investigating further I often restart my machine.
What really happened is the Tablet mode has switched on for no apparent reason. Why it was toggled on Im still puzzling, but you can ensure this doesnt occur in the future by going to Settings > Tablet Mode. Ensure that it always take you to the desktop and when to switch modes, ensure to prompt for confirmation. You can disable Hide app icons on the Taskbar when in Tablet Mode so you can still access the applications youve pinned to the Taskbar.
You can always check and disable Tablet mode from the new Notification menu.
Next time when this full-screen Windows Start menu showing again, no need to panic. Your desktop is still there, and your missing Taskbar is still there. Just need to calm down and disable the Tablet mode.
A few times when this occurred my desktop auto-switched to Tablet mode for no reason and without permission Windows desktop expanded to the second monitor or switch between main displays from the Display configuration. If you too encounter such issue with your Windows 10, this is the way to fix it and bring your desktop glory back.