I lost my retainer a month ago, teeth has shifted..Will I have to get Braces again?

Thank you for your question.Depending on amount of teeth shifting, you might be lucky enough not to have braces again.If it is a minor movement, active retainer might do the job. Another option would be a short term Invisalign or ClearCorrect.In either case you should see your doctor asap.

Depending on how much your teeth have shifted, you may need to have orthodontic treatment. If the shifting is very minor, such as a small amount of lower incisor movement, sometimes it is possible to re-align the teeth by doing a small amount of reproximation (slenderizing). However, this can only be determined after an evaluation by an orthodontist. 

It appears that it is a common thought that impressions or molds of teeth are kept after treatment is complete, including dental care like crowns and dentures.  It is common to hear, "Why don't you just use the mold you used the first time?"Because we don't have it.  We see SO many patients every day that it is physically impossible to keep all the models, especially since it is rare to need to redo treatment.A new retainer will not move the tooth.  A new removable device can be made to move the tooth, but that is not called a retainer.  Retainers retain.If the movement is very small, an aligner, similar in concept to Invisalign, can be made with teeth set straight and hope the tooth moves back, but if significant movement has occured, then orthodontics of some fashion is needed.