Don't get confused by the fact that you're dealing with a weak acid, hydrocyanic acid's reaction with water will still produce hydronium cations,
The only difference between a strong acid and a weak acid is the degree of ionization, i.e. how many molecules of the acid actually ionize in aqueous solution.
In the case of a weak acid, you only get partial ionization, meaning that most of molecules of acid will not donate their acidic proton. To symbolize this, use an equilibrium sign,
The balanced chemical equation that describes hydrocyanic acid's ionization looks like this
#color(green)(|bar(ul(color(white)(a/a)color(black)(color(red)("H")"CN"_ ((aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 3"O"_ ((aq))^(color(red)(+)) + "CN"_((aq))^(-))color(white)(a/a)|)))#
Water is in the liquid state because it represents your solvent. The hydrocyanic acid and the two ions that result from its ionization are in the aqueous state, which essentially means that they are dissolved in water.