Please do first try hard to find it, since the Hauptwerk USB key *is* the licence for Hauptwerk and for all samples set licences installed on it, and if found by somebody else then it could be used in full or re-sold unscrupulously for the value of those licences (which can't be removed/disabled from it, without the possessor's cooperation). This is a relevant excerpt from the 'Licensing and editions' section of the user guide:
Important: Please make sure that you do not lose the Hauptwerk USB key since we cannot normally replace it unless you pay for another license. If you break the USB key we will replace it for a small charge once we have received the original USB key back.
If you are concerned about the possibility of losing the Hauptwerk USB key, software or sample sets, we advise insuring them. Specialist musical equipment insurance policies will usually provide cover for audio hardware, software and sample sets.
Here's one other topic discussing it in more depth:
viewtopic.php?f=1&t=12914&p=95305
If you still really can't find, and if you hadn't insured it, then please send us an email and we can discuss the circumstances privately. (We are very reasonable.)