Windows 10 Reinstallation Problem

[Freezanator]

Ok, I've recently got an SSD and I decided to move my HDD into a caddy to replace my DVD drive. So after downloading the Media Creation Tool from Windows, I put the files in a USB drive and shut down my laptop. After that, I removed my HDD and left my SSD in the main SATA port inside my laptop to install Windows 10 from the USB drive. So, now I'm left with a bare, fresh, clean copy of Windows 10 and I put my HDD back in and my laptop doesn't even see it in Disk Management! I'm totally panicking right now, could someone (preferably a professional) assist me with this problem?

[Altecice]

Can you see the HDD in the BIOS? if you can then it will just need assigned a drive letter from within Windows. (Disk management > select disk > assign > pick a letter)

[Freezanator]

22 minutes ago, Altecice said:

Can you see the HDD in the BIOS? if you can then it will just need assigned a drive letter from within Windows. (Disk management > select disk > assign > pick a letter)

I see 8 options for Boot Priority in the BIOS:
1. SSD [long string of characters] < My SSD
2. Windows Boot Manager
3. USB HDD
4. ATAPI [CD/DVD ROM] < But it has the word 'ATAPI'
5. Internet [something] IPv4
6. Internet [something] IPv6
7. USB [something]

8. USB [something] (7 and 8 are probably USB interfaces)

And now I've got another problem. I can't seem to open Disk Management or File Explorer or even Settings.
I did do something that might have contributed to this. I tried to change the name of the user in regedit from 'freez' to 'Acer' because they went and used the first 5 letters of my email and turned it into my username. (I Googled instructions online)