Events changed by the arrival of the T‑800
Notes on Chronology 2
Let's look into this in more detail to see how the T‑800 failed to kill Sarah. Upon arrival, it gets the addresses of the three Sarah Connors from the phone book and begins killing them. Sarah hears about the first killing while at work. She goes out to the movies and afterward, while having pizza, hears about the second Sarah Connor killing. She goes to the phone, possibly to call Ginger, possibly to call the police, but the phone does not work. She leaves the pizza parlor.
In Terminator, Sarah goes into Tech Noir. She does so in order to avoid Kyle, who is following her, suspecting he may be the one doing the killing. However, there is no Kyle, that hasn't happened. Therefore, Sarah does not go into Tech Noir.
The T‑800 arrives at Sarah's apartment and kills Matt and Ginger. In Terminator it thinks it has killed Sarah until she calls from Tech Noir and leaves a message on the machine, "Ginger, its Sarah!". As she has not gone to Tech Noir, she does not call Ginger and no message is left. The T‑800 leaves thinking it has completed its mission. It probably has instructions on what to do after its mission is over, go somewhere and shut down or go into standby mode. However, it would not pursue Sarah any further as it thinks she is dead.
After Sarah leaves the pizza parlor, she most likely returns to her apartment. Either the police had returned and found Ginger and Matt's bodies or Sarah found them.
One interesting fact is that Sarah never encounters the T‑800. Sarah continues her life unaware that a Terminator had arrived to kill her. All that she knows is that someone killed two Sarah Connors and then killed Ginger probably believing her to be the third. We can only guess how this affects Sarah.
When Sarah, as leader of the resistence, finds out that a Terminator has been sent into the past, to 1984, the events would now make sense. She must realize that the killing of the two Sarah Connors and of Ginger and Matt must have been this Terminator. Even though she survived that encounter, she cannot be certain what the results might be. Sarah has had no time to ponder the intricacies of time travel. She decides to send Kyle back to stop the Terminator.
One last fact, we do not know what year Judgment Day occurs. As mentioned in the Notes on Chronology 1, it is almost certain that Judgment Day occurred after 2004. However, we can be absolutely certain that it does not occur in 1997. This means that when Kyle is sent back, he has some date other than 1997 for Judgment Day and will pass this other date on to Sarah.