I have been using braces for a while. but 2 days after the last check up, one of the lower teeth (middle right) started to shift, not backwards/forward but up. would it be possible to re-position the shifted tooth back to how it was?

Entirely possible. See your orthodontist for an adjustment.

It could be bracket is no longer tied to arch wire, or the bracket has been separated from the tooth. I trust you are seeing a certified Orthodontist. Call him/her immediately, before adjacent teeth slide into that tooth's space. If you are wearing rubber bands, stop until that tooth is relevelled.