If you're also seeing this issue with the default widget libraries in RP, can you try reloading all of the affected libraries? You can do that by clicking the hamburger icon next to the "Libraries" pane and then selecting the "Load Library" option. For Windows, you can find Axure RP's default libraries at the following path:
C:\Program Files (x86)\Axure\Axure RP 8\DefaultSettings\Libraries
If that didn't help, could you tell us what build of RP you're currently using, as well as your computer's operating system version? Any additional information you can provide may help as well. I'll be standing by!