I heard a sad news this morning. A fire broke out at the Notre Dame Cathedral in Paris.

*occur/happen is understandable, but usually fires break out. If it's intentional, we can use "set" as in They set the cathedral on fire.



I have been to there in my twenties. It was so beautiful and I thought that I wanted to stay there for a long time.
*Not wrong, but "that" can also be removed here.


When I left there, I strongly thought I will come back there someday.

*Or: When I left Paris, I was determined to return someday.


Although it occurred today, I will look over their activity for recovering the Notre Dame.

*Do you mean: Although this terrible incident happened today, I look forward to seeing their reconstruction efforts.

Or: I'll keep an eye on the reconstruction of the Notre Dame. = I'll keep myself informed/updated, I'll pay attention to...

:) 

Notre Dame Cathedral

I heard sad news in the news this morning. A fire occurred at the Notre Dame Cathedral in Paris. I went there in my twenties. It was so beautiful and I thought that I wanted to be there for a long time. When I left, I strongly thought I would come back there someday. Although it happened today, I will look over their plans for recovering the Notre Dame.

Notre Dame Cathedral

I heard some sad news this morning. A fire occurred at the Notre Dame Cathedral in Paris. I visited it in my twenties. It was so beautiful and I thought that I wanted to stay there for a long time. When I left Notre Dame, I strongly thought I will come back there someday. Although it occurred today, I will look over the Notre Dame reconstruction efforts.