The "K" series (unlocked) CPUs will work but cannot be overclocked. Also, the VRM (voltage regulator module) on this board is weak—avoid 84W+ CPUs like the i7-4790K without active cooling on the MOSFETs.
Turn off the computer and unplug the power supply from the wall. Acer H81h3-am V1.0 Manual
This is often a RAM seating issue or a failed CMOS battery. Reseat RAM, clear CMOS, and ensure the monitor is connected to the correct port (if using a discrete GPU, do not use VGA/HDMI on the board). The "K" series (unlocked) CPUs will work but
Turn on the computer and immediately tap the key or the F2 key repeatedly until the system setup utility screen appears. Resetting BIOS (Clear CMOS) Acer H81h3-am V1.0 Manual