The problem seems to be that whatever kernel and initrd you have is tied to an old version of CentOS 7 that is no longer in the current repos of most mirrors.
If you were previously able to PXEboot and install CentOS and you are sure your network and tftp are good the problem is that you have an outdated kernel and initramfs that point to a defunct version.
To fix this you need to download the most current version of CentOS's NetInstall ISO, mount it and extract the intird and vmlinuz files.
Mount the .iso
mount -o loop CentOS-7-x86_64-NetInstall-2009.iso mount/
Copy the initramfs and kernel to whereever the old versions are stored and overwrite them (be sure NOT to wipe out your current versions inside /boot!)
#cd to the location of your Centos 7 PXE boot images
#copy initramfs and kernel
cp -a mount/isolinux/vmlinuz .
cp -a mount/isolinux/initrd.img .
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