Did you update from version 4 to version 5? If you did there are special instructions for doing this. If you did you should return to version 4, or the version that you had installed and then go through the update process.
If it was some other version that you went from and to then there may be something else you need to do. In any case it will probably be a good idea to revert to the version that was installed before proceeding.
Did you backup the site before you updated?
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