PS5 suddenly opens a folder full screen. why?  how to fix?

By folder do you mean image?  What version of Photoshop and what operating system?

Try opening Preferences > Interface and uncheck 'Open documents as Tabs'

Please clarify:

1.—Are you talking about the archaically ancient Photoshop 5 or Photoshop CS5, also known as Photoshop 12?

2.— What exact version of either one?  (With all decimals after the dot in the version number.

3.— Photoshop can in no way shape or form show folders; are you talking about the Open dialog box?  If so, the latter is provided by the OS.

4.— What platform are you, Macintosh or Windows?

5.- What exact version of the OS are you running?

6.—Or are you perhaps referencing Bridge?  If so, there's a dedicated Bridge forum:

http://forums.adobe.com/community/bridge?view=discussions

In some instances, it would also be nice to have some details about your hardware. Please read this FAQ for advice on how to ask your questions correctly for quicker and better answers: 

http://forums.adobe.com/thread/419981?tstart=0

Thanks!

Yep. Like Station says, way more information please.  Photoshop doesn't have 'Folders', but you can see folders when using File > Open.  Bridge has folders.  Please try to be clearer.

When I began to type my post, your post with that information had not appeared yet.  Your "as stated" remark is as unwarranted as it is insolent and offensive.

Thanks for clarifying, though,  At least now I know you run Windows, so I can't help you.

I'm out of here.

Grab the bottom right corner of the window and resize it.  It will open to the new size after that.

Double click on the Window Title Bar to get out of full screen.

Blimey RK.  I didn't know that one!