I learned how to do this when i got locked out of my dorm at BYU-Idaho and it works great!! and all it takes is things you can find in your friends room or from the hall.
(the pictures are from different doors because my camera is acting wack, but you should get the idea)
Step 1: Tie floss to paper
take your floss and tie it to your piece of paper
Step 2: Slip paper over the top of the doorway
now take your piece of paper floss combo and slip it through the top of the door. this may take a little finesse, or another sheet of paper to shove it through.
Step 3: Lower paper
Now that you have the paper over on the other side of the door lower it down until you can see it under neath the doorway
Step 4: Grab paper
you should be able to see the paper. if you cannot grab it with your fingers then a captured hanger works great to fish it out.
Step 5: Tie floss to vacuum cable
now that you have the floss through both the top and bottom ends of the door tie the floss to the vacuums cable at least 4 or 5 feet away from the plug.
Step 6: Push cable underneath door and pull on floss
now what you want to do is push the cable underneath the door a little ways. and then gently pull on the floss. you'll want to go slowly so you don't break the floss. but pull on it till you think the cable is about where the doorknob is and try to hook it over the doorknob. this may take a few minutes but eventually you should be able to hook the doorknob.
Step 7: Open the door
once you've hooked the doorknob take hold of both ends of the cable and pull back and forth and try to turn the doorknob. this too can take a few minutes but just push on the door a little and be patient and you should be able to turn the door knob and be back in your room!!