Missing the standing at Arafah entirely means the jamaah haji has missed Haji, as Nabi (shallallahu alaihi wa sallam) said: 'Haji is Arafah.' If a jamaah haji fails to be present at Arafah at any point between the afternoon (zawal) of the 9th and dawn (fajr) of the 10th of Dhul-Hijjah, their Haji is not valid and cannot be completed that year. This is the unanimous position of all four schools of jurisprudence.
Sheikh Ibn Baz explained the procedure for someone who has missed Arafah: the jamaah haji should convert their intention to Umrah, perform tawaf around the Kabah and sa'i between Safa and Marwah, cut or shave their hair, and exit the state of ihram. This converts their missed Haji into an Umrah, which releases them from ihram. If the Haji they missed was their wajib (first) Haji, they must perform it the following year.
Sheikh Ibn Uthaymeen added that if the missed Haji was a voluntary Haji, there is no obligation to make it up. He also noted that a dam (sacrifice of a sheep) is required according to the Hanafi and Maliki schools for someone who missed Haji and converted to Umrah, while the Shafi'i and Hanbali schools do not require a dam for the conversion itself. The key lesson is the absolute centrality of Arafah to the Haji — without it, no other rite can substitute or compensate.