Hi Folks,
Sometimes when we are working on sharepoint 2010 we get a problem in Central Administration showing that as:
SharePoint 2010 – HTTP Error 503 Service Unavailable
To solve that we can do the following ations:
503 ERROR
Q) So what is 503 error in general?
A) 503 (service unavailable).
The 503 Service Unavailable HTTP status code means the web site’s server is simply not available at the moment.
503 errors are usually due to a temporary overloading or maintenance of the server.
Courtesy: About.com
Coming to Sharepoint server 2010
Check the application pool. If it is disabled it caused due to crashes a couple of times You need to manually reenable the application pool in IIS Manager – that should resolves the problem.
Thanks to Stefan Gobner,Microsft.
Bharath Radhekrishna 
Thanks for the post… really helped…
where can i find the application pool on sharepoint
In IIS you can find.
@Josh… Hi 🙂 you can find the application Pool in the IIS.
Go to Start and type IIS in the search bar > click on IIS and then expand your domain name… You will see the Application pool.
Thank you very much … helped me a lot
thank you
Very good, worked man.
Apart from Password expiry and Application Pool in Stopped status, There are few more reasons for Service Unavailable: HTTP Error 503. The service is unavailable in SharePoint
Found this checklist to resolve this error in SharePointDiary.com – http://www.sharepointdiary.com/2011/04/http-error-503-service-is-unavailable.html#ixzz2aWOMfIuk