How do I do this?
If you're able to get into the site yourself, go to the Admin panel and go to Configuration->Server Settings. The option is there.
If you're not able to get into the site, connect through FTP and edit the Settings.php file in the root of your site. You'll see a line which reads "$maintenance = 1;" or "$maintenance = 2;". Change the 1 or 2 to a 0 and save the file.
Julio Alejandro, is this issue resolved now?
The solution provided for you above should fix the problem. I will mark the topic as solved. If you still have a problem with it, feel free to mark it back as unresolved.