Microsoft security essentials won't turn on

Have you tried reinstalling MSE?

Is there a error code that you can post for us to see.

Microsoft Security Essentials works automatically with Microsoft Update to ensure that your virus and spyware definitions are kept up to date.

Open Microsoft Security Essentials, click the Update tab, and then click Update.
If MSE gives you update error message then follow below instructions to Reset the Microsoft antivirus update engine.

1. Click Start, click All Programs, click Accessories, and then right-click Command Prompt, and then select Run as administrator.
2. In the Command Prompt window, type the following commands and press Enter after each command:
   
   • Cd\
   • Cd program files\microsoft security essentials
   • Mpcmdrun –removedefinitions –all
   • Exit
   
   
3. Restart your PC.
4. Open Microsoft Security Essentials, click the Update tab, and then click Update.