I already took my boarding pass and then misplaced my passport
If you have just received your boarding pass - this means you have not yet passed through immigration; however you must have shown your passport to the check-in desk agent in order to receive your boarding pass.
You need to:
First, check your bags. You may have placed it somewhere other than where you would normally look. Try to get to a table where you can put your carry-on luggage and search it it properly.
Check on your person. You may have placed it in a different jacket/pants/coat pocket by mistake. This happens more often than you would think.
Walk back the same way you came and look on the ground, maybe you have dropped it?
Go back to the check-in counter and ask the staff to check. If they found it, they will keep it for you at the check-in desk.
Report the matter to the airport police; they can then see if anyone has turned in a passport to any airport/airline employee.
Unfortunately if all the above fails, you cannot travel as you cannot pass through immigration.
If you have any luggage checked in, ask the airline staff and they will coordinate to get it returned to you. Next, you need to get a police report from the airport police of the incident and take this report to you country's embassy and ask for a replacement passport or emergency travel document.
You'll have to re-book your flight once you have received the replacement passport.