Why can't I restart my computer.

Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.

Then take it in for repair. There isn't anything we can do to help if the computer is dead. All you can try to do at this point is:

Resetting your Mac's PRAM and NVRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

To force shutdown press and hold down the Power button for 6-10 seconds until the computer is off.

Disconnect it from the AC adaptor then remove the battery if it's removable.