Have you tried:
– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switching to the default theme to rule out any theme-related problems.
– renaming the plugins folder via FTP
– reading http://wordpress.org/support/topic/279132
FYI – I am at 2.9.2 and found my widgets sidebar via Admin had disappeared completely… tried switching themes to no avail but using your advice > deactivated all plugins > this worked > reactivated plugins one at a time > this worked…
Found that “Pollin” (http://www.bin-co.com/tools/wordpress/plugins/pollin/) was the culprit…
Thanks!