Magento extensions vary from system to system. With the extension of community edition, the old modules have become obselete for the lastest version i.e Magento CE 1.7.
Sometimes it happens when you install magento extension, due to compatibility issues the magento parser fails to proceed any further i.e if you want install an extension of Magento 1.3 on Magento 1.7, it will stop magento
To Fix this error, we can follow these steps:
1- Firstly, clear all your cache. Remove all the cache file from your temporary folder.
2- Now, in your root folder, there is flag file “maintenance.flag” under your root directory. This file stopes the newly installed module to put direct infulence on the web system. Remove it!
3- Refresh and login your website again.