Depending on your hardware, there are different ways to restart your Macintosh after a power failure or system "crash". (The term "crash" does not imply damage to your computer.)
Power failure restart
Some early Macintosh computers have a slotted power button on the back of the computer. If your computer has this type of power button, you can press it in and turn it to a horizontal position (with the computer upright) to lock it in the on position. The computer will turn on when power is restored.
The Energy Saver and Auto Power On/Off control panels, on later Macintosh computers provide software solutions for power failures. See article 18127 "
Energy Saving Control Panels: Description & Compatibility" for a matrix on which computers can use what versions of the Energy Saver or Auto Power On/Off software.
Notes
1. By default, the Energy Saver will not restart after a power failure.
2. When AppleShare IP is installed, the Energy Saver control panel settings are changed and all server settings are enabled. Choose Server Settings from the Preferences menu in the Energy Saver control panel to see the server settings. (See Figure 1.)

Figure 1 Energy Saver Server Settings window
If you use the Auto Power On/Off Control Panel (see Figure 2), the options are not enabled and you have to open the control panel and click the appropriate checkbox.

Figure 2 Auto Power On/Off window
System crash restart
There are third-party solutions that can monitor your computer and restart it after a system crash. These solutions usually consist of power supplies, software and either an ADB or USB dongle. When the software loses communication with the hardware dongle, a system crash or freeze is assumed and a monitoring software sends a command to restart the computer.
Note: Using the AppleShare IP settings is recommended for restarting the server.
These companies provide monitoring software and hardware solutions:
- APC (http://www.apcc.com)
- Sophisticated Circuits (http://www.sophisticated.com)
Uninterruptible Power Supply (UPS)
To provide power during a power failure, you can use an uninterruptible power supply (UPS). A UPS provides battery backup power during power failures by using a backup battery system.
Note: Information about products not manufactured by Apple is provided for information purposes only, and does not constitute Apple's recommendation or endorsement.