You have written the contents of the file(s) directly to the first partition of the first hard drive on your system. Depending on the size of the file(s) you copied you will have overwritten:
- The NTFS boot sector. Since you say it was not a boot drive, this is probably no great loss.
- The Master File Table. This is a list of where to find all the files on the disk, along with various metadata. Very small files are completely included within the MFT.
- The first few files on the disk. These are permanently lost.
You won't have overwritten the partition table, that lives at the beginning of the disk, outside the partitions. If you had written to /dev/sda (no number) then you would have hit the partition table and master boot record.
To recover your files will be difficult, and making a mistake could damage or overwrite files which are recoverable. It's probably easier to restore from backups - you do have backups, right? Failing that, get a data recovery professional to do it. If you can't do that for whatever reason, make a clone of the drive, and work on the clone. That way, if you make a mistake, you don't destroy the original.
If you are going to try the recovery yourself, you want to get a copy of the MFT. Luckily, on an NTFS system there is at least one backup copy, sometimes referred to as MFT mirror or MFTmirr, somewhere on the partition. If you can find that, then you will be able to copy off the undamaged files.
I have been able to recover a drive in this state (courtesy of a failed windows update) using specialist software. It was a long time ago, and I can't remember what software I used, but it was probably TestDisk. The wiki describes how to restore the MFT from the mirror, and you can get the NTFS boot sector back that way too, if you need it.