All praise is due to Allah alone, and may the Salah and Salam of Allah be upon our Prophet Muhammad, his family and his companions. To answer your question, we say while seeking the success from Allah: The safest view is to start it all over from the beginning. However, if he continued his circumambulation from the round he stopped, it is permissible and by that he would fulfill the obligation. This view is based on the view of scholars that states that ablution is obligatory or a condition for the validity of circumambulation. The majority of scholars believe that it is obligatory to keep short gaps of time between each round of circumambulation; the school of Imam Malik is the strictest of all in this issue.   In any case, if the gap is long then you should start it all over again, especially if the delay was caused deliberately, e.g. stopping to drink tea or heading to a distant place. However, if the person rushed to a place close by, performed ablution and then was delayed whilst returning, he is exempted from the ruling even if the gap between rounds is long.  As for circumambulating without ablution, there is a valid difference of opinion about it among scholars. The view of my Shaykh, Ibn ‘Uthaymeen, was that, if the questioner had not yet circumambulated, he would answer, “Perform ablution and do not circumambulate unless you are in a state of purity” and if the questioner was asking about a past incident where he circumambulated without ablution, he would respond that the circumambulation is valid. This is because there is no explicit evidence that ablution is obligatory or required for the validity of circumambulation. This is also the view of Shaykhul Islam, Ibn Taymiyyah (may Allah have mercy on him) who said, “With no doubt, whoever examined the textual evidence, would know that there is no evidence that ablution is obligatory. However, it is a proven act of Sunnah because the first thing that the Prophet (may the Salah and Salam of Allah be upon him) did when he entered the House was performing ablution and then circumambulating.