I was following a tutorial in blender on basic stuff like nodes, texturing and what not, i tried to hide one of the meshes when I was in edit mode and it went grey. I'm no longer able to edit nor select the mesh. i'm a beginner so you may have to go in depth on how to fix the issue.
You probably don't have the object selected in object mode.
Select the object in the hierarchy (Pictured above, should be on the top-right of your Blender screen). If you wanted to edit the cube, you simply click on the cube in this menu and now you can go into edit mode.
Hope this was helpful!