Waking up from Sleep and the screen stays dark
Hello everyone, I've been having this problem with my mid 2011 iMac. Sometimes (not every single time I do this), when I wake up the computer after being in sleep mode, I hear the normal sound of it waking up but it stays dark. External HD (where I keep the Time Capsule) starts working (I can hear it) and the iMac also sounds like if it was working. The problem is that the screen stays black and nothing changes that, except turning it off (pressing the power button for a while) and then back on. Its starting to get annoying and was wondering if there's anything that could solve this.
Thanks in advance!
PS: Its running the latest version of OSX Yosemite.
iMac, OS X Yosemite (10.10.3), 21.5 inch Mid 2011 model