Try booting other OSes from say the USB (google "how to boot from usb Live OSes.")
The HDD and ODD (hard drive, optical drive) sound like they failed. You can boot and repair many PCs without the ODD but will need a working HDD (or SSD.)
Dell does have some diagnostics, did you try them?
http://www.cnet.com/forums/post/5449d0dd-92c8-44f1-840d-c40d36bdc29f/ notes the D key on powerup.
We are looking for some help with my husband's computer. He has a 2-year-old Dell XPS core i7 desktop running Windows 10 that suddenly will not boot. While he was using it last night, the whole thing suddenly completely froze, so he had to do a hard reboot, but then it wouldn't boot back up. It just sits on the Dell logo screen and won't go any further. He says at one point he was able to get into the bios, but now he can't even get back into them. He didn't even change anything in the bios when he was in them. His DVD drive won't even open. He's been having a problem with the DVD drive for several weeks now in that he could only get it to open by using a paperclip, but even then it wouldn't read any CDs or DVDs at all. We're not sure if this problem is at all related to what is happening now or now. He has been trying to get to the point of being able to format the hard drive and do a fresh install of Windows 10 all day, which he put on a USB thumb drive, but he has not had any luck since he just can't get past the Dell logo screen. Does anyone have any suggestions at all? My husband is quite tach savvy, so he is comfortable with taking the tower apart if he needs to. We are just at a complete roadblock right now and he doesn't know what else to do. Any advice would be greatly appreciated!