With so little information to go on, one can only guess. One possiblity is you're running a version earlier than 1.03 and have installed an add-on that needs v1.03. In that version a variable was added to close a security exploit which newer or updated add-ons check against. If the variable is not present it exits. Try updating vanilla or disabling a recently added add-on (you may need to remove it manually from conf/extensions.php).
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It could, of course, be something else.