It is booting from your Hard Disk Drive probably because of the BIOS boot priority order.
Go into your BIOS (by pressing F2) and go to the "Boot" sub menu.
There, you can find the boot-order. To boot from a CD/DVD/LiveUSB, you will need to move the boot medium to the top of the boot priority list. This is usually done by highlighting the boot device (here, DVD/CD Drive) and then pressing F5 or F6 to move the highlighted device up/down.
Get the bootable medium to the top (in this case, your CD), Save and Exit changes. Your copy of Ubuntu will now boot from the CD. Also try booting from a LiveUSB.
If that doesn't work either, read on :
Assuming that you have a desktop, I suggest you to pull out the CMOS battery for 5 minutes and put it back again. Make sure all your connections are proper.This error is likely due to a hardware failure or loose contact. P.S : If you don't know what a CMOS battery is, take a look here.