If it was completely untied and there is a doubt if it was untied before he said Shema, he must have it retied or borrow someone else’s and say over the first two paragraphs of Shema. If there there’s still some sort of knot even if it doesn’t look like the regular knot and you don’t know when it was untied. It still a good idea to fix the Tefillin or borrow, say over the first two parshiot. So, it is important to say something.
Explanation:
The Gemara in Menachot 35a writes: Abaye said the Shin of tefillin is Halacha L’Moshe MiSinai.” On 35b: Rav Yehuda brei D’rav Shmuel bar Shilat said in the name of Rav the Kesher (knot) the Tefillin is a Halacha L’Moshe MiSinai.”
What is this Kesher?
Rashi explains: One must put the Retzuot (straps) on his head that the forma Dalet so the name of Hashem Shin-Dalet-Yud should be seen on you. The Shin is outdented on the Bayit, the Dalet is in the Kesher (knot) and the Yud is on the knot of the Shel Yad like a small part of the strap folded and sticking out like a Yud.
The Kesher, which is Halacha LMoshe MiSinai, is the three letters of Hashem’s name, and it is the same idea as Abaye, and it is not only the Shin but all three letters.
Tosfot (Menachot 35b DH ‘Elu’) argue on Rashi and say only the Shin of the Shel Rosh is Halacha LMoshe MiSinai. Rav who says Kesher refers to the fact that one needs to have Shel Rosh‘s knot like a Dalet and the Shel Yad like a Yud.
The Keren Orah explains that the new idea here is not that they need a Kesher because without it the Shel Rosh will fall off, but maybe one would think if the Shel Rosh can be secured without a knot it’s fine. The Gemara tells us that no it must be a knot but not necessarily in any special shape.
The Or Zarua (666) Rules like Tosfot that you don’t need a special shape, however since we have the tradition that it is in the shape of a Dalet or a Yud, if it is changed to a different shape it is Passul, because tht is how the Halacha LMoshe MiSinai was transmitted to us.
The difference between the opinions are that if one tied a knot into the shape of a Dalet or Yud then Rashi and the Or Zarua would say it is Passul and you don’t fulfill your mitzvah and Tosfot would say you did. If the knot was missing everyone agrees it tefillin is Passul.
The Shulchan Aruch 3:54 writes: one must pass the straps through the Maavarta and make a knot into the shape of a Dalet on the Shel Rosh and like a Yud on the Shel Yad to complete the letters Shin- Dalet-Yud with the Shin on the Shel Rosh. That is like Rashi’s opinion.
The Mishna Berura 32:233 quotes the Elya Rabah: There was once a story of someone who took of his Tefillin and saw the knot was untied and he ruled they must tie it, put them back on and say Kriat Shema over again without a bracha, since one may put Tefillin on the whole day.” The Mishna Berura explains Rashi since is of the opinion that the Yud is also Halacha LMoshe MiSinai he did not fulfill the mitzvah.”
If the knot became untied after he said Kriat Shema he does not need to put them back on since he fulfilled the main mitzvah, if there was a doubt than he should put them on again.
The reason one goes over Kriat Shema is since if he says Kriat Shema without tefillin it looks like he is lying because it says one must put on tefillin he isn’t. Therefore, one must repeat Shema with Tefillin on. (look in Shulchan Aruch siman 34:2 that if one puts on Tefillin of Rabbienu Tam later he should say the first two parshiot of the Kriat Shema. The Mishna Berura 34:15 adds the last parsha of Shema is about Tzitzit and he does not need to repeat it.)
