Sometimes when I put my iPhone to charge when its dead; it comes on automatically when it is charging, and sometimes I have to physically turned it on.
For example, last night I was on my iPhone, at it died. I plugged it in, then like 40 min after I went to check and I had to turn it back on and it was charged.
This doesn't happen all the time. Most times it comes back on automatically.
"Important: If iPhone is very low on power, it may display an image of a nearly depleted battery, indicating that it needs to charge for up to ten minutes before you can use it. **If iPhone is extremely low on power, the display may be blank for up to two minutes before the low-battery image appears.**". – Joonas Sep 5 '17 at 9:26