Missing the standing at Arafat entirely means the haci has missed Hac, as Hz. Peygamber (sallallahu aleyhi ve sellem) said: 'Hac is Arafat.' If a haci fails to be present at Arafat at any point between the afternoon (zawal) of the 9th and dawn (fajr) of the 10th of Dhul-Hijjah, their Hac 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 Arafat: the haci should convert their intention to Umre, perform tavaf around the Kabe and sa'i between Safa and Marwah, cut or shave their hair, and exit the state of ihram. This converts their missed Hac into an Umre, which releases them from ihram. If the Hac they missed was their farz (first) Hac, they must perform it the following year.
Sheikh Ibn Uthaymeen added that if the missed Hac was a voluntary Hac, 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 Hac and converted to Umre, while the Shafi'i and Hanbali schools do not require a dam for the conversion itself. The key lesson is the absolute centrality of Arafat to the Hac — without it, no other rite can substitute or compensate.