You don't need to get it to boot (that's the whole point). If RAM is loose or defective, or the logic board is bad, the computer may not boot. So, you want to reseat the RAM before attempting to boot. This manual has the instructions. Start on page 7 and read carefully.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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