Right-click on each icon that's 'missing' its picture - then select 'Unpin from taskbar' - so that both are deleted from the Taskbar.
Then left-click on START, Scroll down the A-Z list of apps until you find the 'Microsoft Office' shortcut link (2007, 2010, 2013 or 2016) - left-click on that to expand the list to show each of the shortcuts for the various MS Office apps - Excel, Word, etc.
Hover your mouse over the Excel icon (do NOT left-click on it) - then RIGHT-click over it - select MORE from the pop-up menu, then select 'Pin to taskbar' - that should recreate the icon & it should now display & work correctly.
Repeat the process for the Word icon as well.