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Clicking on any other tool or hitting the escape (esc) key should exit the Crop Tool.  Be cautious of any tutorial suggesting that you can turn a low resolution into a high resolution image.  The reality is that although we can crop pixels, we cannot recreate what was not there.  There are methods to scale an image larger, but that always means interpolating the image.

If you cannot change anything in the layers pallet and are stuck in crop, hit the escape key. (esc)

that will get you out of the active crop mode, which changes the layers pallet to a preview only.

After that, hit V, this will switch you to the move tool, allowing you to continue making any other non-crop changes.

Alternately, you can click on any other tool in the tool bar, changing tools should prompt a dialogue asking if you want to commit the crop changes.  Choose dont crop if you dont want to keep the changes

I agree with Derek.... if the pic is low res, there is no way to make it high res. 

Always work with high res images.  And if you take photos... try shooting them in RAW.  You can always size down when needed.

In fact, I am now getting gallery show offers and some of my older images from years ago will not work as they were not saved as high res.  So to me it is always better to work on high res files as you may not know how they could be used in the future.

You cant really successfully change a low resolution image to a high resolution image.

Re cropping, if you are saying it's a fixed size, you can change this by clicking on the"clear" button in the crop context sensitive menu.