As a follow on from my update above.
You don't say what monitor you have, so I cant give you the exact button to press.
However here is a website for Dell which explains how to set up a monitor.
http://www.dell.com/support/article/us/en/04/SLN128890/EN
If you go to section 3 which is called Correct: Video and Monitor Settings" you see that item 1 says "Set the input".
This is what you need to do, set the input on the monitor so it is looking at the HDMI input.
You probably have not got a Dell monitor so you need to find out how to do it on your monitor.
p.s. It sounds to me like you have bought a new ACER system unit but are using a screen you already had, perhaps a non Acer screen.
But you still need to select the HDMI input on it.