Macos Sierra, Parrallels Desktop & Windows 10

Hello, do you mean your Windows isn't booting to the desktop correctly? I don't know what

the main screen I mean the one with the folders and other stuff on it

is for sure.

Some people have nothing on their desktop - just a picture, and a taskbar- others have a terribly cluttered desktop..

If so, please could you say
- which build of Windows you have (Anniversary/ 14393 / 1607 version) or the current Creator's build (a recent major upgrade)
- what you think happened before this occurred
- whether you see a lock screen (picture) and then when you click on that (e.g.), a login prompt
- where the MAC OS fits into this scenario

Please also complete your specs (see mine for comparison). We don't know if yours is a desktop or laptop, what disks you have, whether you use Windows Home or Pro...
System Specs - Fill in at Ten Forums Windows 10 General Tips Tutorials

Do you have any disk images you have created or system images that can be used to restore your system?

Thanks.

Please give us more information.
Your title sounds like you are running Windows 10 on a Mac running OSX Sierra using Parallels program?.
What Main Screen are you talking about? Windows 10? Or Mac OSX Sierra?

Yes What?
What main screen?
Parallels is a Virtual Machine with Windows running inside of your Mac OS. Here are some FAQ's you should look at if you don't know how it works: Support Installation | Parallels Desktop for Mac