Have you possibly disabled it in Service? Type services.msc in the start box and scroll down the list to "Disk Defragmenter" it should be on automatic, by default, but could remain on manual.
Next, if all is in order there Find the defrag shortcut in the start menu. Right click it and select properties. The "Target" window should say %windir%\system32\dfrgui.exe
If that is in order:
Go to Windows - System 32 and manually click the program (drfgui.exe) and see if it runs from there. If not, you may have a corrupted file. I have attached mine.Unzip and Upload it to the system32 folder on your computer and try again.
