I had braces taking off and now teeth have moved back. I wore retainer day and night constantly. Is there a reason they moved back. Thanks?

Physiologic drift. Not uncommon. Natural physiologic drift can occur because the width of our smiles getting narrower as we get older. See your orthodontist for consultation and future options.
Ortho relapse. When relapse happens even with retainer wear it's usually very easy to restraighten the teeth. Then a different type of retainer system should be used than was used first time around.