if you tubes are only tied not coterized it is possible they came undone
Having your "tubes tied" can mean a variation of surgeries.. If you had clips inserted they can loosen and migrate from the area, if youve had silicone rings inserted they can snap or loosen.. Both of these techniques cause scarring on the fallopian tubes, so even if the procedure is reversed it causes scarring making it difficult to get pregnant. Another way iof tubal ligation is Electrocoagulation or cauterization which by the saying "tubes tied" Im guessing you havent had this done.
It is never impossible to fall pregnant with any form of contraception, if it's not done correctly there is a chance. But the chances are so slim, even if the clips/rings have moved there will still be significant scarring and depending how long youve had them in the implant often becomes incased in body tissues making it harder to move.
Pregnacies that result as a failure of tube ligation need to be investigated as they can result in ectopical pregnancy (where the baby grows in the fallopian tubes)
I suggest visiting your GP for a pregnacy test and a scan (just for piece of mind) to have a look whether the clips/rings are still hanging in there!