Connect the flash drive you made with Windows 10 Media Creation Tool or the Dell Backup tool, and then press f12. Boot from that flash drive and follow the installation prompts.
Did the system every boot normally after you installed the new drive, and how did you get the OS on it?
This type of thing is hard to analyze without the actual system. There may be several different causes but the update had a mention of external devices causing problems with missing boot drives, so if you have any external devices, try removing them and them go back and check your bios to make sure the correct device is set as primary boot device.