Some people say that it is possible for a blackwhole to evaporate, if so then where does all the matter that fell into it go?

There's a lot of theoretical information on this topic and conjectures as to what could happen, but based on what we think we know, if a black hole did evaporate and something had gone into it, it too would evaporate. I would recommend reading this book by Neil deGrasse Tyson, an incredible astrophysicist.

This webpage/article from Berkeley provides a good deal of information about black holes.

Stephen Hawking proposed the theory that black holes can emit radiation, now called "Hawking radiation," which causes them to lose mass over time. A black hole can evaporate completely or lose mass gradually as a result of emitting Hawking radiation. The theory also states that matter falling into a black hole is assumed to contribute to the mass and energy of the black hole, and that mass and energy are released back into the universe when the black hole evaporates.

According to theoretical physics, black holes can undergo a process called Hawking radiation, where they gradually lose mass over time. As black holes emit this radiation, they lose energy and mass, eventually evaporating completely. The matter that fell into the black hole is thought to be released back into the universe as part of this radiation during the evaporation process. This concept is based on quantum mechanics and general relativity, but it has not been directly observed yet.