Magento shows this error when you install Magento extensions. There might be two reasons of this error.
Check below steps:
1) Go to the Magento root installation, find a maintenance.flag document and erase it using FTP or Filemanager. After this, Magento will not display maintenance mode message. If you still face the problem, delete your sessions and cache to make one more attempt to solve the issue.
2) Call your hosting company to fix the issue.
