All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Reciting Soorah Al-Kahf [Quran 18] on Friday is a recommended act of Sunnah and not an obligation. There is no harm in resuming the recitation of the Soorah from the verse at which you had stopped. You are not required to recite the whole Soorah from the very first verse. It should be noted, though, that in order to obtain the full reward mentioned in the Hadeeth, one should recite the whole Soorah each Friday. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever reads Soorah Al-Kahf on Friday will have a light that will shine for him from one Friday to the next.” [Al-Haakim and Al-Bayhaqi - Al-Albaani and As-Suyooti graded it Saheeh (sound)]
When one recites part of the Soorah, it is not considered the same as reciting the whole Soorah (in terms of reward).
Please, refer to Fatwa 129804 on the fact that the more effort the person exerts while reciting the Quran, the greater the reward
Allaah Knows best.