I performed Umrah. I forgot to cut my hair after I finished. I put on my clothes and remembered the next day. What should I do? What if I slept with my wife?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

You have to wear the dress of Ihram immediately after remembering this matter and then shave your head completely or shorten your hair.
According to the preponderant opinion of the Muslim scholars you are not obliged to pay any expiation to make up for this mistake, since you neglected shaving your head due to forgetfulness and you had sex with your wife assuming that you became Halal (finishing Umrah). Allah Says (interpretation of meaning): {… And there is no sin on you concerning that in which you made a mistake, except in regard to what your hearts deliberately intend. And Allah is Ever Oft-Forgiving, Most Merciful.}[33:5].
Imams Ibn Majah and al-Baihaqi reported from Ibn Abbas that the Prophet said: "(Almighty Allah forgives my Ummah's mistakes, forgetfulness, and the acts on which they are forced)".
Allah knows best.