Why does my steak turn out tough?

I cooked a steak in a pan last night and I didnt cook it very long ir was very pink inside and it was still tough. What am I doing wrong? Is there anything i can do to fix it once its tough like crock pot it or boil it or is it just ruined. I was told to just cook it a few minutes on both sides then let it sit in the microwave (turned off) and it will continue to cook itself