Thanks, Nabeel, I just went ahead and reinstalled Office, which worked fine.  Unclear what caused the issue; the only SW mod was the windows update. The update troubleshooter in MS website corrected 2 "errors" in the previous update. No idea why, but all clear now.
I just installed a Seagate Backup Plus drive (purchased from Dell) on my XPS system running Win 8.1.  Installed backup SW that came with the drive. Now I cannot open MS Office 2013 apps (Outlook, Excel, Powerpoint), but no problem with any other app installed on the PC.Checked file location that the program icons point to, and the .exe files are missing! (FYI, that's C:Program Files>Microsoft Office 15>root>Office 15>OUTLOOK.EXE, etc.)Now that I think of it, I had a Windows Update installed automatically yesterday, which may have caused the issue.Anyone seen this issue, and is my only solution to reinstall the apps?.  I do not like such major corruption to occur for unknown reasons.  Who knows what else is lurking there?
MS Office 2013 EXE files missing