delete the following file from your /magento/ installation
e.g. rm ./magento/maintenance.flag
This helped me.
I believe that when I installed an extension, it only went part of the way or there was an error in it.
It got to the point of putting my site in maintenance mode but didn’t bring it out of it after the install
If after the remove of the flag file you get an 404 error and this occurred after you tried to uninstall an extension with connect, then you should go to val/log and check what exactly causes the error.
Probably something isn’t deleted properly, so delete manually and reload the website.
This will make the trick
Sindre|ProperHost - 16 March 2011 10:49 PM
There are two main reasons why you may get this error.
1. There is an actual problem with the server, resulting in the 503 error response. In this case, you should contact your hosting provider.
2. Your store has been put in maintenance mode. If Magento sees a file named maintenance.flag in the root directory, it will automatically redirect all requests to the default 503 error page. This can be used during upgrades, etc. to prevent users from accessing the store. The solution is, as already mentioned, to delete/rename the maintenance.flag file.
There might also be a good idea to create a more user friendly “maintenance page” than the 503 error, which is kind of misleading for customers.