HTTP Error 503. The service is unavailable - Solution 2
First of all try our Solution No 1 in the following link.
HTTP Error 503. The service is unavailable
If the above steps didn't solve the issue, please try the following steps.
From the list of Application Pools, Right click on your Application Pool.
Then Select Advanced Settings.
Now a new window will come as shown below.
Here under Process Model Section, select "Load User Profile".
The value of Load User Profile is of type Boolean.
For Resolving the Issue, just change Load User Profile from True to False.
And then Click OK Button. After that Restart your Application Pool.
Now if you browse the website, the error will be Resolved.