My mac is on, but the screen stays black. Help

There's another possibility for a dark screen, but only for the non-Unibody models with removable main batteries (2006-2008, Models A1150/A1211/A1226/A1260). These generations also have a replaceable PRAM battery, which helps the computer keep track of the time and screen resolution settings while the computer is powered off. When this battery weakens with age (which usually takes several years), the computer may exhibit one or both of these symptoms:

• on powerup, the computer will make the normal startup sound, but the screen doesn't light up
• the clock will be way off, resetting to a time like Jan 1, 1970

Since the default Date & Time setting for later generations of OSX is to synchronize time automatically from an Internet-based network time server, it's possible to have a weak PRAM battery for a long time without detecting it. If you have suspicions. you can switch your Date & Time system preference to manual, then shut down the computer and cold-start it, to see if the clock loses the manual setting.

If it's a PRAM problem, the dark-screen problem is a result of the failure of the computer to send a sync signal to the display at startup. Sometimes you can get the display to light up by starting up with the dark display, then doing a force-restart (Command-Control-Power). The computer may retain enough of a charge from the first startup to supply a sync signal for the restart.

If this is the problem, and you have a non-Unibody model that has a replaceable PRAM battery, you can get the parts and the replacement guides here. In Unibody/Retina models, PRAM power is drawn from the main battery, which remains in the computer at all times.

MacBook Pro 15" (Model A1150) PRAM Battery

MacBook Pro 15" (Models A1211, A1226, A1260) PRAM Battery

MacBook Pro Model A1150 PRAM Battery Replacement

MacBook Pro Model A1211 PRAM Battery Replacement

MacBook Pro Model A1226, A1260 PRAM Battery Replacement