A coworker recently had something like this occur. They copied the text of an entire document, went to paste it into a new document...and it was gone. When I got the call, I shared this tip with them.
To find the clipboard manager, look on the 'Home' tab of the ribbon on the far left, and click on the box in the corner:
This will display the clipboard pane. If you want Office to collect clipboard items whether the clipboard pane is visible or not, click Options at the bottom of the pane and select 'Collect Without Showing Office Clipboard'.
Items on the clipboard will survive until you clear them or until you close all Office programs.
If you are working on a low-resource (RAM) machine, you can also open up more memory by clearing your clipboard through the clipboard pane.