Another posssible reason is that the feature you are trying to use won't work right now. For example, Ctrl+T won't work if you don't have any layers selected or if you are using the Crop tool and have not yet accepted the crop. Or perhaps you have a dialog box open.
The next time a shortcut doesn't work, try doing it from the menu. In the example of Ctrl+T, go to Edit > Transform to see if it's available or grayed out.
Post a screen shot when it happens and we might be able to spot the issue.
I opened a photo which I made before. I selected the text layer and press Ctrl+T to resize the text. There is a small error screen come into view. This is all occur after I purchase new version of photoshop