This question was asked in another thread but it was closed so couldn't reply there. Failure to boot past Dell logo can be caused by individual peripherals failing. I recently had this problem and it was the keyboard causing it. Unplugging all USB devices allowed the PC to boot and then it was just a case of plugging them all back in and restarting the PC multiple times, each time unplugging a single USB device until the computer booted and the culprit was found. Hope this helps people.
Dell desktop won't boot past dell screen
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