Why did my phone randomly turn off?

[AC Question]

I have a telstra phone and earlier today at 5PM it randomly switched off. It's now 7.32 and it still won't turn on. I've been clicking the power button and then still won't turn on. Anyone know what to do?

 

[B. Diddy]

Welcome to Android Central! Which phone? Telstra is your carrier, not the phone brand or model.