I was trying to install Ubuntu (the installation went well) while already having installed windows 8. After installation I wanted to change the boot option back to hard drive but this option wasnt there, only CD/DVD option. Now I am stucked in BIOS not knowing how to boot from hard drive. I have ASUS N56V laptop.
You probably didn't make the drive bootable for one reason or another during the Ubuntu installation.
You probably can hit F12 or similar on your BIOS to bring up a boot menu, which should let you select the hard drive even if it won't let you set it as a boot device in the BIOS.
Or, you can boot off of your Ubuntu install disc and run it in Live mode.
Once in Ubuntu, bring up the Disk Utility, select Edit Partition, and make sure "Bootable" is checked.