Difference Soft Reset/Hard Reset Just a quick question. 
I have been trying to figure out the difference between Soft Reset, Hard Reset, Factory reset. If anyone could please help me to clarify the differences, it would be greatly appreciated, thank you. 

A little bit of random information. So my phone was in my pocket, seems that it was reset (Turned off and instantly came back on) by the power button being held down for fifteen to twenty seconds, wondering if this is considered a Soft Reset, or Hard Reset, or if there really is a difference.
Soft Reset is simply powering your phone off and on by using power button - no data is lost
Hard Reset is forcibly stopping the power to phone by removing the battery ( if the battery is user replaceable)- no data is lost


Both these are first level means of rectifying minor issues and are harmless


Factory Reset is restoring the device to the state it was when you purchased it (with few exceptions) - all user data is lost. This is next level of troubleshooting or happens when you unlock bootloader. See also What is hard reset and what is factory reset?


What you describe is not any of the above. It is sleep state or sleep mode which your device goes into when it is inactive for long. This is a battery saving measure. One activates or wakes up the device  by power button or home button  ( if it was soft reset, it would have displayed start up logo and process)

Edit: OP clarified that start up logo appeared so it was actually a soft reset . Mobile likely got switched off accidentally