What happened between Abigail Williams and John Proctor prior to the opening of The Crucible?

The text makes it obvious that John and Abigail (who was then employed by the Proctors as a maid) have been involved in an extramarital affair. In Act 1, when the two of them are alone in the room with the unconscious Betty, Abigail makes an advance on him. She is upset when he resists her and says that their liaison is over. She then reminds him of the times that they have actually been together:

I know how you clutched my back behind your house and sweated like a stallion whenever I come near! 

Abigail makes it clear that she believes that it was John and not his wife, Elizabeth, who dismissed her from their service. She states that she knows that John still loves her just as much as he did before her humiliating dismissal.

It’s she put me out, you cannot pretend it were you. I saw your face when she put me out, and you loved me then and you do now! 

Elizabeth terminated Abigail's service when she suspected the affair.

Abigail's claim that John still has a soft spot for her is supported by his acknowledgement that he does sometimes look up to her window when he passes her residence. Elizabeth later makes a similar accusation in Act 2 when she first states that he still blushes when Abigail passes him in church. She also later claims that John refuses to speak to Abigail or denounce her since the teenager still has a hold on him.

You’ll tear it free--when you come to know that I will be your only wife, or no wife at all! She has an arrow in you yet, John Proctor, and you know it well!

It is tragically ironic that Elizabeth later lies in court about the affair. Thinking that she is protecting her husband's integrity, she testifies that she dismissed Abigail because she felt that John was paying her too much attention, not because of his adultery. 

I came to think he fancied her. And so one night I lost my wits, I think, and put her out on the highroad.

She then tells the court, on a direct question from Judge Danforth, that John did not turn away from her. What Elizabeth does not realize is that John has already testified that he was a lecher. Her confession leads to her husband's arrest for contempt and eventually results in his conviction as one who had been conspiring with the Devil.