Edit page is blank (5.5.2.1)

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I have an old install (5.5.2.1) that a client has used flawlessly for 3 years now.

Suddenly, they can't get to the edit screen, it is just blank. The dashboard appears as normal and seems to function fine. However, when you try to view a page from the Sitemap - it just comes up blank with no controls on the screen at all.

Any ideas would be greatly appreciated. Incidentally, I've restored a one-month old backup from the server (which I think precedes the problem), but still no go.