My application was working fine yesterday. I started my PC today. When I tried to start Magento I got this error message.

Service Temporarily Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

I am not even able to open the admin page or see any sort of errors on the page. If I visit any URL of my Magento site I am getting the above output. How do I resolve this?

Check if there is a file called maintenance.flag and if so delete it.

Magento 1.x : maintenance.flag file is in : magento root directory

Magento 2.x : maintenance.flag file is in : var folder

When Magento is performing certain tasks it temporarily creates this file. Magento checks for its existence and if it's there will send users to the page you described.

It's supposed to automatically delete this file when done processing whatever task it was doing, but I've experienced occasions where something went wrong and it failed to delete it.