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I have two possible fixes to try:

1) Simple launch the game and then pressed alt+enter.

2) Or try to locate the game's "config file", in your 'Documents' folder you should have a folder called 'Ghost Games', go into it and then the NFS-Rivals folder. Youy will see a settings folder, in it a file called 'PROFILEOPTIONS_profile' should be seen.  (The whole path is like this: Documents/Ghost games/Need for Speed(TM) Rivals/Settings )

Open that file using Notepad, then if you scroll it's contents you will see various audio and video settings. Amongst the settings check if you have 'FullscreenEnabled 1'. If you have  'FullscreenEnabled 0' <--- zero, please edit the Zero to One.  (0 means dissabed, 1 means enabled.)

Save then, run your game and see if that helps. I think other parameters can be changed this way as well, but I wouldn't play around as it saves with settings changes in game.

Hope this helps!