iPhone restarting then blacks out

In mid text my iphone restarted, it stayed for near 15 seconds then afterwards, nothing. It completely blacked out. Normally the side button and home button needs to be held down to restart however I noticed I only needed to press the side button and it'd restart however I was met with the same outcome. 15 second apple logo then black. I've heard that one of the solutions may be connecting to to a computer and using iTunes? but as it restores it'll wipe out all the data. Could there possibly be anything else wrong? Another solution was replacing the battery but I don't know. My other question is, if restoring does indeed wipe out the data and you have to back it up do you need iCloud in order to back up things such as photos, videos, etc? I have no idea if I was logged into iCloud in the first place as it was a phone passed down from my aunt. I was signed into iTunes and the App Store though. I had a lot of important stuff on there..