ASP.NET Application support and debugging

Question

What are the best practices /steps to investigate if an ASP.net web application hosted on IIS with SQL server is not responding.

 

from programmer prospective i do start with the exact error message details.

 

I would like to know for an application available in production and if i have to start troubleshooting from production. 

 

What if there is no specific error message (a blank page / delay in loading the page or may be while submitting or requesting for a specific action on a page).

 

Starting from IIS to SQL and then debug the same in VS if the issue is due to code,

 

 what are the basic things i should be checking in IIS as a programmer ?

 

 so that i can check if its SQL(including sql sp and trigger) via profiler or the source code (the c# code for business logic ) via VS.

 

Its been asked to me multiple times in interviews, and i always felt confused myself on this.