my emac won't start up

DOn't hold the power button. Hold the Command S down while booting. This will put you into single user mode.
When all of the text is finished, type fsck -fy

put a space after the fsck.
As it checks the disk, it may find problems and fix. If it finds problems and fixes them, run it again. When it is done, type
reboot

This will fix many hard drive directory problems. If this doesn't work, you have to boot from the system CD and do an archive and install. This will put a new system on your computer, but save your old documents.

How much free space was there on the hard drive when the Mac was last used? Had it given any warnings about the hard drive being full?

Begin by unplugging all peripherals other than the Apple USB keyboard and mouse. Continue with the step-by-step suggestions in Your Mac won't start up in Mac OS X. If you have your OS X Install disc at hand, you can boot from that disc and run Disk Utility from the Installer Menu, selecting Repair Disk on the internal hard drive. Minus the OS X install disk, you can as advised use the Unix file system check utility fsck. Both are described by FSCK.

You maight also try to Start Up in Safe Mode. Hold the Shift key down until the grey screen / Apple logo appears, then go read a magazine or something --- it can take upwards of 20 minutes for Safe Mode to finish running a faceless variant of Repair Disk in the background. That time was for a 40 GB HD with about 26 GB on it; if you have a larger drive, it could take even longer.