How to recognize external drive and fix NTLDR?

Hi there!
My computer is running on Windows XP and just recently, I backed it up for the first time using Microsoft backup installed with the operating system.
The thing is when I restarted my computer, I got an error message saying that the NTLDR is missing, I tried to repair this by using the recovery disk in my computer but when I did, my computer recognizes my external hard drive as the C: drive.
I need to know how I could get my computer to recognize the internal hard drive as the C: drive, rather than the external one.
I already ran the diagnostics on the internal HDD and it seemed to be fine.
I hope you can tell me what’s going on and how I can fix it. Thank you.
