A verb in the Qenya Lexicon of the 1910s appearing as ᴱQ. hilkin “it freezes” based on the early root ᴱ√HḶKḶ, a variant of ᴱ√HELE (QL/39).
Neo-Quenya: Since there is no sign of ᴱ√HḶKḶ in Tolkien’s later writings, I think it is best is adapt this verb as ᴺQ. hel- “freeze” based directly on the later root √KHEL of ice words. Since the root is ice-related, I’d limit the meaning of this verb to the freezing of water and similar substances. For freezing weather, I’d use Q. nicu-.
The verb G. hel- “freeze” appeared in the Gnomish Lexicon of the 1910s as derivative of primitive χele-k (GL/48), based on the early root ᴱ√HELE as suggested by Christopher Tolkien (LT1A/Helkar; QL/39).
Neo-Sindarin: Since √KHEL appeared in later writings with glosses like “freeze” (Ety/KHEL) and “ice” (PE17/116), I think ᴺS. hel- “to freeze” can salvaged for Neo-Sindarin.