[Dave]

1. You can't restore it to a point in history without backups I'm afraid. As for the session table, if this is the only missing table then you're lucky because it only stores (non-critical) data temporarily. Execute the following query in a MySQL console, through PHPMyAdmin or ask your host to execute it against your database:

PHP Code:

CREATE TABLE vb_session (
    sessionhash CHAR(32) NOT NULL DEFAULT '',
    userid INT UNSIGNED NOT NULL DEFAULT '0',
    host CHAR(15) NOT NULL DEFAULT '',
    idhash CHAR(32) NOT NULL DEFAULT '',
    lastactivity INT UNSIGNED NOT NULL DEFAULT '0',
    location VARCHAR(255) NOT NULL DEFAULT '',
    useragent VARCHAR(255) NOT NULL DEFAULT '',
    styleid SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    languageid SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    loggedin SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    inforum SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    inthread INT UNSIGNED NOT NULL DEFAULT '0',
    incalendar SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    badlocation SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    bypass TINYINT NOT NULL DEFAULT '0',
    profileupdate SMALLINT UNSIGNED NOT NULL DEFAULT '0',
    apiclientid INT UNSIGNED NOT NULL DEFAULT '0',
    apiaccesstoken VARCHAR(32) NOT NULL DEFAULT '',
    isbot TINYINT NOT NULL DEFAULT '0',
    PRIMARY KEY (sessionhash),
    KEY last_activity USING BTREE (lastactivity),
    KEY user_activity USING BTREE (userid, lastactivity),
    KEY guest_lookup (idhash, host, userid),
    KEY apiaccesstoken (apiaccesstoken)
) ENGINE = MEMORY 


It's entirely possible that more tables are missing, but just execute this first to find out if it's the only table that's missing.

2. I wouldn't do that. I would first fix it so your current vBulletin 4 forum is working and then upgrade to vBulletin 5.

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I don't see how a MySQL upgrade could cause this though, your host did something very wrong. If this happened to me, I would move host right away.

Also just a tip: keep offline backups, on your own computer or on a different server so you will not have such issues in the future.