For unfamiliar words, it never hurts to check the dictionary. Here is what Lewis and Short have:
Īō, Iūs, and Īōn , Iōnis, f., = Ἰώ, I.a daughter of Inachus, king of Argos, beloved by Jupiter, and changed, through fear of Juno, into a cow; afterwards worshipped as an Egyptian deity, under the name of Isis. —Form Io, Ov. H. 14, 85; Prop. 2, 28 (3, 24), 17; Ov. M. 1, 588 sq.; Val. Fl. 4, 351 sq.; Hyg. Fab. 145.—Gen. Ius, Nemes. Cyn. 31.—Acc. Io, Ov. M. 1, 588; Amm. 2, 19, 29.—Abl. Io, Prop. 2, 13, 19.—Form Ion; dat. Ioni, Plaut. Aul. 3, 6, 20.—Acc. Ionem, Serv. Verg. A. 3, 153.
Annoyingly, the reference is slightly off, but here's the Ovid passage in question (Ov. Met. 1.583-585):
Inachus unus abest imoque reconditus antro
fletibus auget aquas natamque miserrimus Io
luget ut amissam.
A. S. Kline translates it thus:
Only Inachus is missing, but hidden in the deepest cave he swells his stream with tears, and in utter misery laments his lost daughter, Io.
You'll see that, like pulchram in Thirty-Eight Latin Stories, here amissam is your clue that Io is in the accusative case. The context makes that clear.