It is exactly the same as for any other lost ticket. As long as you have the ticket number (and having a photocopy of the ticket is more than enough for that) you will have no problem, particularly if you can go to the office of the airline issuing the ticket (otherwise it may be a 24 hour delay). They will invalidate the lost/stolen ticket and issue a replacement (same routing and terms). If your ticket happens to be used prior to you reporting it then it gets sticky -- you are liable to pay the cost of the used segments (but how likely is that these days, with security checking that passport and ticket matches? all the instances that I have heard about were clearly pre-arranged fraud in shady 3rd world countries). They may require a police report. If you want a refund instead of a replacement ticket (why?), the normal refund rules apply except any payment is delayed for 90 days (waiting for the stolen ticket to clear the system from siberia or some other obscure location).