It happened to me several times. In the past (and up to a point even now) my girlfriend was not used to my size.
She often asked me to stop because it was painful to her, or because i hit the cervix, and so on.
Obviously it was VERY disappointing, but sex is for two, and i love her, so i always immediately stopped.
Sometimes i just went down on her and give her some oral, just to make her feel some pleasure and make the penetration easier, and then tried again.
Not always the solution worked: in those cases we suspended the intercourse.
She often asked me to stop because it was painful to her, or because i hit the cervix, and so on.
Obviously it was VERY disappointing, but sex is for two, and i love her, so i always immediately stopped.
Sometimes i just went down on her and give her some oral, just to make her feel some pleasure and make the penetration easier, and then tried again.
Not always the solution worked: in those cases we suspended the intercourse.