iPhone 8 broke down

Two weeks ago my iPhone just turned off and I couldn't turn it on. Nothing helped. I brought the iPhone used maybe 3 months ago (but I set it in my setting beginning of February) and went to the apple store for a case and a screen protector. Maybe my phone don't like the heat? It only happens when I'm in warm cities... but it's so annoying because I travel and everything is on the phone. After 2 hours, sometimes faster I can turn it on again. Sometimes my phone turn itself off and when on. I need help...

Best,

Saskia