I can't get my computer to go to sleep. It will go manually.
What OS? Check the power settings in control panel is windows. A device may be preventing your computer from sleeping.
This gives little to not much info to know what to say??? Laptop or desktop and O/S????
First check the BIOS - it will show you the options available...
If you're using Linux, some ad-hoc distros does not have the sleep mode, try to check on the distro official site for more info.