It sounds like you are either going into and out of tablet mode (something 2-in-1 devices can do), or you have changed a Start Screen setting in Personalization. Here are guides for both situations, I imagine you'll quickly know which one applies to your device:
What is Tablet Mode in Windows 10 and How to Turn it On and Off
How do I get rid of the full screen Start screen in Windows 10 | Windows 10 content from Windows IT Pro
Make Start full screen - Windows Help
Let us know if those guides don't hellp
What is Tablet Mode in Windows 10 and How to Turn it On and Off
How do I get rid of the full screen Start screen in Windows 10 | Windows 10 content from Windows IT Pro
Make Start full screen - Windows Help
Let us know if those guides don't hellp