I plugged my ps4 into an acer monitor, and it says cable not connected. I took everything out and plugged it all back in, but it still says that every time I try it.

Probably incompatible with the monitor. Could also be a bad cable.

What model PS4, what model monitor, what type of HDMI cable? It's possible there is an incompatibility if the monitor is an older model and doesn't support the newer HDMI standards. It's also possible that there might be something wrong with the HDMI port on your monitor. Do other devices work when connected, and does the PS4 work when connected to other displays using that same HDMI cable?

Huh... Making some guesses here: assuming HDMI from the ps4 and then right to an HDMI on the monitor. Monitors usual automatically detect. Is there and input switch?