I had a tongue piercing for awhile, but now I took it away. Just like you I wanted to have it back so I went to the local piercing studio and let them do it, it's just like the first time you take it, just easier - Since they'll already have a hole to stick it it. So get the piercing-shop to do it!
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If the hole has only shrunk minimally then it is possible to simply push the jewellery through, stretching the hole slightly and reopening it. This should only be done with smooth jewellery - do not try this with an externally threaded barbell (if your jewellery has an ending which resembles a screw, don't use it to stretch).
However, if this is more than a little painful then the hole has closed too much to be reopened. Allow the piercing to heal completely and see your friendly neighbourhood piercer to have it re-pierced.