I am using a desktop OSX version 10.7.5. Each night I exit my desktop icons and put the system to "sleep" (not shut down). Normally my computer springs back to life when I touch the mouse, space-bar or any other key. Over the last few days the computer has not re-awakened when I press the mouse or a key. I have to switch off the computer using the button at the back of the screen - and then re-boot the system from scratch. This takes time for everything to re-load and I do not know why it has suddenly started happening - for no apparent reason. Can anyone help please ?