Misplaced my cert.

Assuming you're talking about a med. certificate, you need to get another one. It must be carried with you when you are excercising the privelages of your pilot certificate.

You have two choices- call the FAA and get a copy faxed or mailed to you, or just get a new medical from an AME (probably more expensive, but much less hassle, no doubt).
You can get one by filling out the lost certificate form on the FAA website. It will cost you $2.00 to replace and I am not sure how long it will take to get it to you.
I just had that exact problem.

Had my wallet stolen, and lost my airman certs as well as my medical cert. I called the FAA at 405-954-3261, and told them that I needed a fax of my certs (airman & medical). They faxed it within the hour, and that fax is good for 60 days. It's a simple piece of paper - just has your name, cert number, ratings, class of medical, etc.

In the meantime, you can go here and get the address to send the $2 and have them send you a replacement copy of your medical.
If they are still local you can go to your AME and they will type you up another one with the same effective date. That's what my roomate did.