The test pyramid, black versus white box, severity versus priority, and the STLC vocabulary asked verbatim.
3 questions in QA & Testing.
Work out what the test was actually asserting. If it pins behaviour a user depends on, the refactor changed behaviour and the test did its job; if it pins an implementation detail, it never had value and the failure is the cost of having written it that way.
Split the input domain into classes the code should treat identically, then test each class once and probe every boundary at the smallest step the field allows. Exhaustive testing is ruled out by combinatorics, which is why a coverage number is a claim about your model rather than about correctness.
Line coverage tells you which lines executed while the tests ran, which is not the same as which behaviours were verified: a test with no assertions still counts, and the interesting information is in the uncovered lines and in whether the tests would fail if the code were wrong.