Is there an alternative way to reset an iPhone?

Hello, I hope someone can help me. I installed the new oven thermometer app from the AppStore and it seemed to work fine the first time for my bread baking (225ºC/435ºF).
But now the iPhone is acting a bit strange, for one the the screen looks really strange stuck on one color (gold) so I think it maybe needs a reset with the on/off button, but that's not working properly either, well actually I can't seem to find it anymore. Is there another way to reset an iPhone instead of the on/off button?

Please help! Because I really need it to work again, I need to bake more bread tomorrow.

Thank you for any advice or tips...

P.s. as per the warning in the instructions of the app, I made absolutely sure not to turn on the grill function of the oven...