How can I use my TV as a screen?

I want to use my old little screen as a screen for my laptop. I plugged it in via HDMI and set everything to project, but my screen only displayed "no signal" all the time (I was also on the screen on the HDMI input). Can someone help me please?

The LG 19LS4R may have to be switched to another input.

Here: https://www.lg.com/.../lg-19LS4R