O2

My Samsung phone screen is 'frozen'. I know the solution is to hold two side keys down together to force it to restart but I can't remember which two. Help, please.

My phone is fully charged and still beeps to let me know I have texts, e-mails etc, but it does not work when I try to react to these messages. The screen is unresponsive. I can use the side button to call up the 'power off' screen but none of the op...