Dell S2415H monitor not recognizing macbook

My partner's Macbook (Retina, 12-inch, early 2015, running High Sierra) has abruptly stopped working with our Dell S2415H monitor. It currently says "There is no signal coming from your computer."

It worked for a few months and then inexplicably stopped. I wasn't there when it happened, so I'm not sure what changed or if there was an OS update, although my partner can't recall anything changing. I have switched out all of the cables and adapters and the problem persists. I tried hooking it up to my PC via HDMI and it worked instantly with no installation. I have made sure that the monitor is set to HDMI input, which is what we are using. Currently I am connecting her laptop using a standard HDMI cable and an HDMI to USB-C adapter. I pulled up the display settings on the macbook and tried the "detect display" button and nothing changes there either. The monitor just stays in power saver mode when her laptop is plugged in.

Anyone else had this problem? I saw a long thread about another Dell monitor that used a DP cable and needing to disable some DP settings, but this monitor only has HDMI and VGA ports.