Getting 'Object Reference not set' error when editin

We're running Sitefinity 3.7 on one of our sites.
I don't know what's recently changed apart from content in pages, I've been reassured nothing structural, but all of a sudden whenever we try to edit a page in the admin, we get an error "Object reference not set to an instance of an object."
The front-end still looks fine, and the back-end navigates fine, we just can't edit anything.
I don't know where to look in SF for more info or details.

Any clues?

(I know this is a very old version, but it's been working fine for a long time and "just upgrade it" is not really an option to fix a spontaneous issue.)

Hi,

If all controls placed in the backend page return object reference then the .NET framework version has been changed either on the server were the site runs or the application pool uses different .net version now. Refer to this KB article to check the current version of 3.7 site and then see what .net version have to be used for the application pool running the site if there have been changes.

Regards,
Stanislav Velikov
Telerik