I fixed this database connection error by finding my config.php file and editing it - you don't have to switch database's or do a new board. its quite simple, find the original config.php file and rename it config2.php or something so you have a backup of the original. Then, edit the original config.php file and save it as config.php - just keep the config2 for backup incase this doesn't work...now, make sure your sql server address, username, and password are all correct. I ran into problems because my website provider changed my sql server so all i had to do was update my sql server address. I don't know what else could be the problem, but don't give up - you will find answers in this forum GUARENTEED! search, post, and just wait! Good luck! let me know if this works!
ps - after you used another persons config file, its realizing yes you do have a database, but you receive these multiple errors because none of the core information is right (sql location, usernma,e passwords, etc). If you are using someone else's config which is specified to THEIR forum, not yours, it will screw a lot up.