Exhcange 2013 OWA 404 error

I no longer have access to OWA from a web browser. I get "HTTP Error 404.0 - Not Found" after entering login details. However clients on the LAN are able to connect to Exchange, Mobile clients can connect remotely on port 443 (Mostly iPhones) and Exchange EAC works in a browser?
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IvanSystem EngineerCommented:

you mean no1 can access owa anymore or just some users? Do you get to a login page or not?
Have you installed some RU or SP, changed certificate, any errors in Event Viewer after you try to access owa?

cmdlnAuthor Commented:
Nobody has access to OWA anymore and yes as I mentioned you do get the login screen. I have not installed any SP but I do keep the server up to date. I don't see anything in the event log at the time of log in but I do see a lot of the following:

Event 5011, WAS

A process serving application pool 'MSExchangeOWAAppPool' suffered a fatal communication error with the Windows Process Activation Service. The process id was '832'. The data field contains the error number.
cmdlnAuthor Commented:
I think I may have stumbled onto something. OWA is missing from the Exchange backend site. See the attachment. Can I manually create this somehow?
cmdlnAuthor Commented:
Ok was a bit of a hack but basically this is what I did to get this working again. I tried to run New-OwaVirtualDirectory to create the missing application in IIS, but it threw and InvalidOperation error. I then manually added the application owa to "Exchange Back End" in IIS. This did not resolve the issue but it did allow me to run

Remove-OwaVirtualDirectory -Identity "Contoso\owa (Exchange Back End)"

I could then run

New-OwaVirtualDirectory -WebSiteName "Exchange Back End"

Once this was done I could login as normal.

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