Hello Abdullokh,
Thank you for your questioin about a 503 error in Magento. Here is a link to a similar question we answered titled What is a good troubleshooting method for Magento 503 errors?.
In that answer, we discuss the maintenance.flag file, which is put in place when you install extensions.
If you have any further questions, feel free to post them below.
Thank you,
John-Paul