What do I do to make this Dell OPTIPLEX 210L boot up?

Try booting into bios. If successful check to see if bios recognizes your hard drive. If you can't boot disconnect all external devices except the keyboard and try again. If you still can't boot try replacing the cmos battery. If you still can't boot you probably have a motherboard problem. If you can boot try to safe boot the computer by pressing F8 while booting. If you can safe boot then you most likely have a software problem. Boot with your system disk and run system repair. Finally if you can boot to your system disk and see your hard drive and all else has failed format the drive and reinstall the operating system.