First, you should ask her what problem she is facing in our realtion. If she replies that everything is fine and alright then you should ask her is she hiding something from you, if she says no everything is normal and why are you behaving in odd way then tell her that you saw her with the other person in office. If she realises what she had done then tell she broke your trust and failed to be loyal, ask her what she want mor in your relationship, why she was with the other man. Give her one more chance and try to mainatin the relation again, spend more time with her, show more care and affection towards her. It would be better if you will handle this clamly, without shouting on her because this will leave impression on her mind that how calm and loving my husband is  then why I was doing wrong to him. Behaving in normal way will be difficult for you but, try and give her time to realise her mistake. This will preserve your bond and will show diffrence on her.