If you are uncertain what you've done, first stop is a repair of Internet Explorer.
Go to the Start Menu, Settings, Control Panel.
Open Add/Remove Programs and find "Internet Explorer x.x and Tools" or something similar. The x.x is the version number.
Select it and choose Add/Remove (or Remove or whatever it is).
From the box that appears, choose Repair. Then wait. You will need to restart your computer after.
This may not solve it, but if it's a missing file or setting, then this will pick it up.