There will usually be a single line of text error message when installing an unsupported, but nominally compatible CPU (e.g. newer BIOS needed to support it, but it is electrically compatible otherwise). There are a couple of things to check first, like whether that particular CPU is compatible with the motherboard and if it is, whether there's any damage to the pins in the LGA socket, causing it not to make solid contact with the CPU's pads.
But it could also be something else like the memory isn't compatible, or is installed incorrectly.
Dell has a penchant for getting rid of support for some pre-production ES steppings as soon as the production steppings are finalized. What sometimes works is using a current production processor and reverting the system to the earliest known BIOS for the system (usually A00 in Dell's case). Install the ES and see if it boots. Proceed upgrading one BIOS version at a time until the version that removes the early ES support is found, reinstall the production CPU, revert the BIOS one version and then reinstall the ES.