Today I was flying at around 400 meters going up, and the drone started going above 500m with no input to the RC. I got to about 600m even when pushing the stick down. Then I pressed the land key and stick down, and it finally went down. I was scared that I was going to lose my spark.Do you guys have an idea what could have caused this?

Hello there lakijs1992. I am sorry for the trouble this has caused and thank you for reaching out. May we please know before having this issue, did your DJI Spark had any hard landing or a crash before? In addition kindly please try to calibrate your DJI Spark's remote controller for us to be able to isolate the issue. Thank you.

Hi lakijs,

Just so you know: for "Open Category" drones like the Spark you are limited to a maximum of 120m in Latvia (like generally in EASA airspace). Read the rules please:
Drone Laws in Latvia

If the air was foggy/hazy it could very well be the downward sensor interpreting that haze wrongly as ground approaching, and so the drone tried to climb more.

HTH,
Wolferl