Hell is the eternal separation from God.
The person who dies in the state of mortal sin has deliberately torn himself from God during life and has died without that bond of union with God which we call sanctifying grace. He has no means by which to establish contact with God and has lost Him forever. He is in Hell.
We know that in Hell there is everlasting fire, because Jesus Himself said so. It is not the kind of fire we see on earth; earthly fire cannot afflict the soul, since the soul is a spirit. There is in Hell a “pain of sense” of such a nature that it cannot be better described by any other word in our human language than by the word “fire.” What matters most is the “pain of loss.” The eternal separation from God constitutes the worst of Hell’s suffering. It will be extreme loneliness.