Things that break tayammum
1 – Everything that breaks wudu or necessitates ghusl breaks tayammum, too. For instance, if the nose of a person who is on tayammum bleeds, his tayammum is broken.
2 – If the state of excuse that makes tayammum valid disappears, tayammum is broken. For instance, if water is found or the barrier or excuse to use the water disappears, tayammum is broken.
Three situations are in question when water is found or when the barrier or excuse to use the water disappears:
a) If water is found or the state of excuse disappears before a person starts to perform the prayer, tayammum is broken at once. He has to make wudu for the prayer.
b) If water is found or the state of excuse disappears while a person is performing the prayer, both tayammum and the prayer are invalidated. The person has to make wudu and perform the prayer again.
c) If water is found or the state of excuse disappears after the prayer is performed, tayammum is broken but the prayer does not have to be performed again.