Sometimes Magento gets stuck in “Maintenance mode”. It means that there is maintenance.flag file in Magento’s root folder.
The standard maintenance mode of Magento is a bit “too universal” – it sets Magento backend (admin) to maintenance mode also. Once you’re in maintenance mode, it’s hard to get out of this if you don’t have access server’s shell.
Anyway – there is one option if you have not removed Magento Connect Manager (a.k.a /downloader). This program is be impacted by the maintenance.flag file. Log in to Connect Manager at /downloader and check/uncheck checkbox “Put store on the maintenance mode while installing/upgrading/backup creation“.

That’s it.