Imac (2010) won't wake up after installing yosemite

Question

blue screen, with lots of disk activity but won't wake without a hard restart. This is rough on the system (or will need to turn off sleep mode). Is there a fix for this that anyone knows of, or must we wait for an update? don't have the problem with my macbook air (2011) - thx

Answer 1

Step 1
Take all the applicable steps in this support article. That's the starting point for any further efforts to solve the problem. Please read the whole article carefully (some of it is hidden by disclosure triangles) and don't skip any of the steps, including the ones under the heading "Additional troubleshooting." If you don't understand any of the steps, ask for guidance.
Step 2
If you're running OS X 10.8.5 or earlier, from the menu bar select
           ▹ System Preferences... ▹ Accessibility
If the checkbox at the bottom marked
          Enable access for assistive devices
is checked, uncheck it and test.
If you're running OS X 10.9 or later, select
           ▹ System Preferences... ▹ Security & Privacy ▹ Privacy ▹ Accessibility
If any applications are listed on the right and have a checked box next to them, uncheck all the boxes and test. You may first have to click the padlock icon in the lower left corner of the window and authenticate as an administrator to unlock the settings.
Step 3
In the Energy Saver pane of System Preferences, uncheck the box marked
          Put hard disks to sleep when possible
Step 4
This and the next step must be taken as an administrator. If you have only one user account, you are the administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
In the Console window, select
          DIAGNOSTIC AND USAGE INFORMATION ▹ System Diagnostic Reports
(not Diagnostic and Usage Messages) from the log list on the left. If you don't see that heading, select
          View ▹ Show Log List
from the menu bar.
There is a disclosure triangle to the left of the list item. If the triangle is pointing to the right, click it so that it points down. You may see one or more reports with a name that begins with "Sleep Wake Failure" followed by a date. Select the most recent one. The contents of the report will appear on the right. Use copy and paste to post the entire contents—the text, not a screenshot.
I know the report is long, maybe several hundred lines. Please post all of it anyway.
Step 5
In the Console window, select
          DIAGNOSTIC AND USAGE INFORMATION ▹ Diagnostic and Usage Messages
In the search box in the window's toolbar, enter "powerd" (without the quotes.)
You may see one or more lines like this:
com.apple.message.signature: Platform Failure
If you do see that, and you did everything in the preceeding steps, then you're dealing with an internal hardware fault. Make a "Genius" appointment at an Apple Store, or go to another authorized service provider.