I agree with Beytan Kurt.
I had 503 thrown for both the Central Admin site as well as the SharePoint landing page. In both cases the Passwords were expired.
After resetting the password in the AD, and refreshing the Identity, CA worked but the SharePoint landing page threw a 500 error.
It turned out that the .Net Framework Version was set to V4.0. I changed it to V2.0 and it worked.
Remember after each change you need to recycle the appropriate app pool.