The page showed this error:

Server Error in '/dotnetnuke' Application.

 Could not load file or assembly 'System.Net.Http.Formatting' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

After some head-scratching and experimenting, we determined that the web.config was looking for a different version of the System.Net.Http.Formatting dll than the one we had in the bin folder (which appeared to have been changed somehow). After trying a few things,  we were able to bring the site back up, by replacing 'System.Net.Http.Fomatting.dll' with a version from a different instance of DNN we have running on the same webserver. We also had to do the same thing for a couple of other dll's  - (it was basically a  case of getting the dll's version to match the version that the web.config was looking for.)

Also, once the site was back up, we noticed the Personna bar was missing!! After some Googling we were able to replace that by restoring the 'Newtonsoft.Json.dll' file , which was missing.

I've seen one or two threads about similar issues, but I have no idea why this occurred. It seems as though , for some reason, some dll's were changed in the bin folder, and one removed - for no apparent reason - causing the website to crash.

Does anyone have any insight into why this may have happened, and how to prevent it from occurring in the future?

Thanks
Tom