Changed clock setting System won't boot

I guess you mean the CPU clock setting, and not the date/time settings, as these would not prevent the computer from booting.

Usually you can get back to the BIOS "fail safe defaults" by pressing a certain key or combination of keys as the computer comes on. With many BIOSes it is the Insert key. You may well need to borrow a non-USB keyboard & plug it in correctly to do this. (Connect the keyboard while The PC is switched off!)

You may need to do it a couple of times before you hit the right moment.

Also, read your motherboard manual to find out how to "reset the CMOS".

Once you have done this you can go into the BIOS and change the settings to "optimized defaults" as the fail-safe ones might well give you sluggish performance.

this is really easy to get back. i would put the battery on the system board back and reset the jumper on the system board for CMOS. also re sit the processor. this will detect that a cpu is avaliable and it will boot. once booted it will ask you to reset the time and date. and also the serial ID #. when this comes up enter the SN# of the unit. depending on the unit.