If the computer will not even begin to start up, or it will not recognize its ports, then follow these instructions from Apple to reset the System Management Controller (SMC) in Intel-based Macs or Power Management Unit (PMU) in PowerPC-based Macs:
If your Mac has been running fine but has suddenly begun to act up without your doing anything to it, here are the first things to try.
If you are an ordinary user, and if your computer is working fine for other users, then quit all applications, run Yasu with these four checkboxes ticked off, and log out:
If a single application is failing and this does not fix it, then the problem is among the files created by that application and owned by you. Probably a file in "~/Library/Preferences" has got corrupted and needs to be deleted. To find out which file is likely to be causing the problem, consult the troubleshooting section of that application's manual or web site.
If the computer will let you log in, then quit all applications, run Yasu with these 13 checkboxes ticked off, and restart:
If this does not fix the problem, boot from a DiskWarrior CD and rebuild the disk directory.
If there is still a problem, and if it involves erratic freezes and kernel panics, test the computer's RAM with memtest.