Windows 7: can't reopen minimised window on taskbar, preview OK
Try hold Windows Key + Shift then press the Left Arrowkey 2 or 3 times. If that doesn't work try again with the Right Arrowkey instead.
I solved the issue whereby only one icon in the taskbar won't maximize/restore:
Right-click the icon to see the pop-up menu with options (close window, pin to taskbar, name of program).
Click on the name of the program. This should result in the taskbar icon flashing.
Press and hold Win, and browse through opened windows by pressing Tab. You should have your software maximized again. If this doesn't work, do the next step.
Win+D (pressing these keys together should refresh your desktop to original positions).
Here's how I fixed the problem.
I had one WordPad document open and it was stuck minimized in the taskbar.
I tried all the other suggestions here without success.
Because I am using Windows Classic theme, I thought to switch to an Aero theme so I might at least get a hint of what was on the WordPad document by using Aero Preview.
As soon as I switched to the Aero theme the WordPad document maximized.
So, try switching themes types (Aero -> Classic) or (Classic -> Aero) and see what happens.