External ASUS monitor goes to sleep

I have a MacBook Pro and an external ASUS monitor connected to it with HDMI, and have had this setup for a couple of years. A few weeks ago I started having a problem I had not experienced before. My MacBook Pro will go to sleep, and when I wake it up, the ASUS monitor is black; I assume it has gone to sleep. I have discovered that if I restart the MacBook Pro, the problem is solved, but nothing else I have tried seems to make any difference. Turning the ASUS off then back on makes no difference. I tried using a different, identical HDMI cable, and that made no difference. If I select "About this Mac" and "Displays" the ASUS shows up, but if I try the Displays tab in System Preferences it doesn't seem to know that the ASUS exists. I suspect this is a software problem rather than a hardware problem, but I haven't added any new software to my Mac in the last two or three months, and I have only experienced this problem for about the past month. The ASUS works fine most of the time, but occasionally the screen goes blank, and nothing does any good except to restart my Mac.