What happens in you project context when a bug is caught in production?
What kind of things, in your opinion, shouldn’t happen?
What kind of things, in your opinion, should happen?
Attempt to reproduce it. Write out the steps to reproduce. Friendly devs fix it. Test it. Support inform everyone who experienced it. Get everyone involved together for a post mortem meeting (5 whys etc). If deemed necessary, write a formal post mortem. For me, one of the most important parts of it all is to stop people feeling guilty, or stressed, or in any way bad about it. A post mortem is a learning document.
panic…
Read the customer ticket, and Inform the parties that need to know soonest.
Don’t : Spend loads of time trying to repro the problem, you don’t own the defect, the team does.
Do : Try and understand how the defect arose for yourself as a tester, go learn more about the environment and the use-case that caused the unhappy customer. Place the defect in a context, and then scan the vicinity for similar animals hiding around the same spot. Finding and Dealing with these at the same time may save time now.
its painful sometimes 
first we check the priority, if the bug is critical/blocker first thing we do is plan a hot fix to roll-out . at this situation no point of trying to find how this happened if its clear what need to do to fix the customer problems.
then do a further analysis to understand what went wrong.