There are as many answers to this question as there are people on this planet. Did she cheat once, or was it an ongoing affair? Did she cheat with someone she knew, or with a stranger? Were there multiple affairs? Is she sorry? Was there a tangled web of lying, or a one night mistake followed by a confession?
You said ex -- so how long have you been apart? Do you miss her? Do you love her? Is this relationship worth risking getting hurt? Are there other factors involved, like children?
Tell us more if you want more guidance here, but ultimately this is your decision.