The retainer bonded behind my teeth came off after my orthodontic treatment.

Do not remove the loose retainer yourself - come in as soon as you can, within a few days, to have it re-bonded. If it stays attached on one side only, the wire can poke your tongue, and the teeth that treatment straightened can shift again. The first year after treatment ends is when teeth have the strongest tendency to return to their old positions, so extra care is needed. If the wire has come off completely, keep it from being bent and bring it with you so we can check whether it can be re-bonded.