If the shel yad became loose
If the shel yad loosened before the shel rosh was put on, it may be tightened without a new beracha. If the shel yad or shel rosh loosened at a later point, they should be tightened with a new beracha. According to the Rama (who holds that a new beracha is not required when the tefillin are removed with the intention of replacing them), a new beracha is not required if they loosened during the course of davening.
(סעיף יב וס"ק נא-נב; וראה ביה"ל ד"ה אינו)