I'm sure this question is a duplicate, but I can't find it, so I've answered it.
TL;DR: The historic singular "you" was lost and plural "you" took its place.
Early Modern English had singular "thou"/"thee" and plural "ye"/"you". Plural "ye" was also used to show respect to a single person, as "thou" was a term of familiarity.
- "thou": singular and familiar;
- "ye": singular formal, or plural.
Singular "thou"/"thee" was replaced by
plural formal "ye"/"you" and became obsolete, as did the verb conjugations that went with it. Eventually, "ye" merged with "you" losing "ye" in Standard English giving us the current situation.
Note that the first person pronouns still have four different words for subjective and objective: I, me, we, us, but because of the two historic mergers that only affected "you", the second person pronouns now only have one for all of them: you.
From Wikipedia's article on the word you:
In standard contemporary Modern English, you is both singular and plural; it always takes a verb form that originally marked the word as plural, (i.e. you are, in common with we are and they are). This was not always so. Early Modern English distinguished between the plural ye and the singular thou. As in many other European languages, English at the time had a T–V distinction, which made the plural forms more respectful and deferential; they were used to address strangers and social superiors. This distinction ultimately led to familiar thou becoming obsolete in modern English, although it persists in some English dialects. Because thou is now seen primarily in literary sources such as the King James Bible (often directed to God, who is traditionally addressed in the familiar) or Shakespeare (often in dramatic dialogues, e.g. "Wherefore art thou Romeo?"), it is now widely perceived as more formal, rather than familiar. Although the other forms for the plural second-person pronoun are now used for the singular second-person pronoun in modern English, the plural reflexive form "yourselves" is not used for the singular; instead "yourself" is used for the singular second-person reflexive pronoun.
You is derived from Old English ge or ȝe (both pronounced roughly like Modern English yay), which was the old nominative case form of the pronoun, and eow, which was the old accusative case form of the pronoun. In Middle English the nominative case became ye, and the oblique case (formed by the merger of the accusative case and the former dative case) was you. In early Modern English either the nominative or the accusative form had been generalized in most dialects. Most generalized you; some dialects in the north of England and Scotland generalized ye, or use ye as a clipped or clitic form of the pronoun.