to make matters worse another problem occurred to my iBook. I can't access to OS X Leopard anymore. After a few system reboots using the button (the OS was stuck and I couldn't shut it down normally) the iBook shows the Apple logo on the grey screen, the spinning wheel and it keeps loading forever. I've already installed Tiger on another partition to access my files but I can't access to leopard anymore. I've tried repairing permissions using utility disk but nothing.
The first thing I'd try is
booting into Safe Mode.
Here and
here are more things to try if your iBook can't get there.
You may need to delete the Tiger partition; having it there could inhibit your ability to select the Leopard partition. This sounds weird - even to me - but in my experience, sometimes your Mac needs to have only one choice; otherwise it will always default to the "good" one. Another reason to drop the Tiger partition is that trying to repair permissions on a Leopard install from Tiger will probably not have the desired effect; this function takes quite a bit longer on Leopard than it does on Tiger, it's only reasonable to assume that Leopard is checking/repairing some things that Tiger doesn't even look for.
If you just can't get into Safe Mode in Leopard, the next thing to try is
Target Disk Mode. If you have another Mac running Leopard or later, see if you can boot your iBook into TDM; then connect via FireWire and troubleshoot your iBook's Leopard install from the other Mac. This can also be done the other way round (booting the other Mac into TDM, and booting your iBook from that) if the other Mac is also running Leopard.
EDIT: One more thing. In my experience, sometimes Leopard just needs to take its time "thinking." I've had a Leopard install hang in the way that you describe and, reaching the end of my troubleshooting abilities, have decided to just leave it on and spinning overnight while I slept. On a couple of occasions, I awoke the next morning to find the Mac booted into Leopard and waiting. This may have something to do with what
@AphoticD suggested above; I've never tried the "fsck" repair in single user mode, it's possible that if I had I wouldn't have had to keep my Mac up all night. But Leopard is a pretty forgiving OS, sometimes it just needs time to take care of things itself. Good luck!