Can't access files on computer

At the Command Prompt, when I try to change the drive it says “The System cannot find the drive specified.”

Windows does not always assign the same drive letter to different devices.  And sometimes it does.  It’s just what Windows does.  I have a couple of USB drives that get assigned the J: drive letter and similar ones that get the S: drive letter and sometimes they switch.

Look at Windows Explorer, see what drive letter is assigned to the USB drive then use that in the DIR command. DIR D:\ or DIR E:\ or DIR F:\ etc until you see a list of files and directories.  No need to “change the drive”, the DIR command will run just find from the C: drive.