“1969” is episode twenty one of season two of Stargate SG-1.
SG-1 step through the stargate and end up under a Titan missile that is being test fired, without a gate. Being under a missile is not a healthy place to be.
There was a solar flare which has caused them to travel back to 1969. Problems arise because they are intruders in a secure facility. Further problems relate to how they get back.
General Hammond knew it was going to happen, and gave Carter a note before they left, and they meet a, much younger (and junior), version of him, who got the note and understood it.
This is the first time that time travel has occurred in the franchise (and it gives them the chance to explain all the paradoxical problems with time travel). It became quite a recurring thread in other episodes, and series, and was sometimes rather confusing, especially in Stargate Universe.