I've been having this issue for months with my HP monitor. It just went in to sleep mode as soon as I turn my computer on. I googled and searched many videos on youtube but nothing I found work. I changed the monitor cable etc,it still wouldn't work. I eventually tried this final method of un-plugging and re-plugging and it brought the computer out of sleep mode. P.S I'm not a computer technician just a regular person who've been having issues with the monitor. This worked for my situation, hopefully it works for yours too.