I uncheck the "offline mode" box in the Virtuemart backend but the site still displays the offline message. I have tried adding define('PSHOP_IS_OFFLINE', ''); to the config file but no luck. Any ideas?
I assume you are using VM2.0.18a on Joomla 2.5.9?
Have you cleared the browser cache and cleared the Joomla cache - in Site/Maintenance.
Make sure that in joomla_root/administrator/components/com_virtuemart/virtuemart.cfg you have the line shop_is_offline=0 under ##shop settings
If nothing works then you can reset the configuration - under Tools, Renew VirtueMart configuration by file