I have a MacBook Pro 13" (2013). After it goes into sleep it does not wake up. I see the tabulator key light goes on if I press it, but the monitor stays black. I can only restart the Mac by pushing the on/off key.
I had this problem and did everything everybody recommended, including a clean reinstall. I found a workaround- Empty the Caches daily or every other day. Turn on Enable the Develop Menu Choose Safari > Preferences, and click Advanced. At the bottom of the pane, select the “Show Develop menu in menu bar” checkbox.
I do not know why this works but it does for me. And if I forget to empty the cache for an extra day I have the gray screen is back.