Q: Vertical blue lines on start up on mac and then frozen screen

Hi AmyMegLaura,

Thanks for visiting Apple Support Communities.

If you are seeing unexpected behavior on your Mac or third party display, try these troubleshooting steps first:

No video or no signal, image distortion, "snow," or flickering

Check connections

When using an external display be sure to check the following:

1. If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
   
   
2. Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
   
   • Monitor and Display Adapter Table
   • About Mini DisplayPort to HDMI adapters
   • Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
   • Thunderbolt ports and displays: Frequently asked questions (FAQ)
     
     
3. Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
   
   
4. Try unplugging the video adapter or cable and then plug it back in.
   
   
5. If more than one video adapter is in use (or "daisy-chained"), troubleshoot by using only one adapter.
   
   
   • Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
     
     
6. If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).

Reset the system
You can reset the Mac's parameter RAM and SMC.

Reset the resolution

Start by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:

1. Start up in Safe Mode.
   
2. From the Apple () menu, choose System Preferences.
   
3. Choose Displays from the View menu to open the preferences pane.
   
4. Select any resolution and refresh rate that your display supports.
   
5. Restart your computer.

You can find these steps and more information here:

Apple computers: Troubleshooting issues with video on internal or external displays
http://support.apple.com/kb/ht1573

Best,

Jeremy