With Lufthansa, it's not possible to retrieve the PNR/reservation online from the e-ticket number alone (very few airlines offer this facility). I just tried the Swiss site suggested above, but with a pure Lufthansa itinerary, and it can't retrieve it from the e-ticket alone. For another itinerary involving both Lufthansa and Swiss, the Swiss website was able to provide all details from the eticket number alone.

You'll have to call (or visit) the agent that made the booking, or ring Lufthansa.