Most of the time this error occurs due to the maintenance mode, when it is enabled while installation. You can easily resole this error if you have access of root directory of your Magento installation.
- Login to cpanel.
- Click on file manager under “Files” section.
- Here you have to click on public_html folder and search for maintenance.flag file.
- After that you have to remove this file from your account and try to access your website.