Some things to check:
- Is the database up and running?
- Is the database accessible?
- Check your your ../db/local.php file.... Is it readable by Tiki? Is it corrupt? Is the database information correct?
- Can you re-run the installer (tiki-install.php) and reset your database information? (Do NOT replace the database, though!)
- Did you review your server error logs to see what caused the problem?