If you get nothing when trying to start up you need to open it and check the diagnostic lights to the left of the RAM slots. Please let us know what they indicate. You may have lost your power supply. If it does tone when trying to start then start up from your system installation disk. Go to the second screen. Pull down menu Utilities -> Disk Utility. You should see the hard drive icon in the left hand column. Click on the hard drive icon to select it and run disk repair till it runs clear. Now try restarting. If the drive is un-repairable, do a write zero's format of it to map out an bad blocks and then reinstall the system.