If you ran the built-in Windows Disk Cleanup Utility, CCleaner, or other cleanup programs, this will often remove the recently used items. Note that these are built-in shortcuts to the real documents and you can still search on them and open them again. The more you open the files, the more they will appear as being recently used docs. Try running a search for the files you are looking for, or even some of the text in the document.
If this doesn't work, right-click on the Taskbar, go to Properties, click on the Start Menu tab:
Make sure the checkbox is enabled for:
Store and display recently opened programs in the Start menu
Store and display recently opened items in the Start menu and the taskbar
There is also further instruction here if the jump lists/menus have become completely corrupted: