That all depends on the spell and whether it was an important aspect of the spell.
If you are doing a candle spell that requires repeated lighting and work on a daily basis till the candle is burned out, then you should be fine, as the candle has burned out, just sooner than expected.
If you are partially through a set amount of time (such as a 3 day spell) then you may have to get a new candle and pick up where you left off with that one, or start again with a larger candle..
You should also bear in mind that despite these candles being long lasting or week long candles, any prolonged burning, (past four hours) at a time can shorten the burn time quite considerably, this is because the glass it is sitting in is heating up and melting wax below the flame. If you are planning to use a glass candle in the future it is best to keep burning time in mind to allow it to last the expected time.