Are you sure the poets haven't consented? We haven't seen your contest solicitation but every writing contest we have seen provides some rules about rights, particularly if a selection is chosen as a winner or runner-up. Did your solicitation contain any statement that could arguably imply permission? Even if not, you may be able to argue that furnishing an entry implied that the winners expected to be published and consented by way of an implied license. We can't guarantee that argument will work in your case but it would succeed, for example, when a letter to the editor is published. (Implied licenses and related permission are discussed in this poetry-related entry.) (Certainly, your future contests should contain a statement that winning entries will be published.)
Should you publish? We can't tell you whether to go ahead without permission. As a general rule we would be more inclined to proceed without permission for an online, electronic anthology than for a print publication. That's because the online entry can be easily taken down if there is a complaint, while the print publication is irreversible. In any case, if you can't find the poets online, it's equally possible that the poets won't find your anthology, in which case, the risk is diminished substantially.