Ran into this one today… and it is nothing you want to see happen on an Exchange server! I was re-assigning services to a new SSL certificate when I started getting this error. After doing some searching, I found the easy fix from Microsoft. Under Server Config > Client Access, you can click Reset Virtual Directories on the far right. From there, you can choose which directories to rebuild (OWA, Autodiscover, etc.) Doing that fixed my issue for OWA!
More info can be found in Microsoft’s Technet Article: https://technet.microsoft.com/en-us/library/ff629372(v=exchg.141).aspx