The entity framework creates a model of your database, a .edmx file. If you open the file and close it, then you can’t open it again until you close visual studio and reopen it. That no fun.
Here’s a work around, right click you edmx file and choose Open With…
You will get a choice of what you want to open the model with. If you choose XML Editor you will be able to open just the xml. (If may warn you that your model is already open and will ask if it can close it, choose yes).
Now it’s open, but you see all the XML.
Right click on the edmx file again and choose Open.
Your model will open back in the familiar view that your used to.