Q: If we do not see the moon on what should be the 1st night of Shawwal (30th evening of Ramadan), what should we do?
A: If you began fasting for Ramadan based on a single witness or due to the sky/horizon being obstructed, you must continue to fast the additional 31st day. It’s likely that you began fasting 1 day early.
If you began fasting Ramadan based on 2 witnesses and have completed 30 days, you are to break your fast the following day regardless if the sky is clear or obstructed.
Source: Hidayat al-Raghib