I'm not sure about the answer to this.

It could be a number of things.
- View menu > Screen Mode.
- Possibly setting Workspace to default in Photoshop's settings.

If neither of those things helps, then make a note of all your settings and then set everything to back default.

You might need to restart Photoshop to see some of the changes such as settings changes.

* Joe, if you wish to post about Photoshop it is probably best you remember to post in this particular section of the forum: Aid to Digital Photography. Perhaps even bookmark it.