1
In the end of the film, everyone was crying.
At the end of the film, everyone was crying.
Use at the end (of something) to refer to the final point of a particular place, period or event: ‘at the end of the street’, ‘at the end of the week’ and ‘at the end of the film’.
2
I waited for more than one hour and, at the end, I left without her.
I waited for more than one hour and, in the end, I left without her.
Use in the end to mean ‘finally’ or to say what happened after some time, difficulty or thought: ‘In the end, we decided to stay at home.’
In the example above, we are describing what the speaker finally did after waiting, so we use in the end, not at the end.
See BEGINNING