I'm not sure how I did it but I accidentally deleted the mainframe and don't know how to bring it back. while I was trying to, I duplicated the thing under the mainframe and put it above it. I have tried so many things to get It back for so long. I noticed that when I went to another project or a new one it was like usual. Please help I am extremely frustrated. Here are some pictures to show what I am talking about if you got confused:
It is not called a mainframe, it is called the Outliner. This took me a while to figure out what you meant, but that's fine.
Look very closely at the top right corner inside where the outliner should be. See that gray arrow pointing down? Click it or drag it down. A bar should slide down. Go to the left side of that bar, and there is another dropdown. Click it, and from the window types select Data->Outliner.
Click the arrow to unhide the header and switch to Outliner.