I've obviously done something stupid while editing a .php file to remove the "Congratulations, you've installed Zen Cart...."
Now the shop has disappeared!
The file I was altering was the "includes/languages/english/classic/header.php" file, specifically the line which goes "define('HEADER_ALT_TEXT',..."
I changed the text, saved it and bingo....no more shop!
Any ideas how to rectify this situation or do I just remove and reinstall the entire shop (rather drastic!)?