Your explanation sounds as you have exported all the tag-field values into a text file, but have left out the full filepath to the original media file, which is the binding relation between text and media file.
Provided you have the backup text file, with each line having a list of tag-field-values and also the full file path included, then Mp3tag can easily do the import of the tag-field values to the related media file.
The backup text file can be of any sorting, because Mp3tag can bind each data line to its related media file due to the full file path pointer.
If some target media file does not exist, then Mp3tag skips the current text line and reports an error, but does the import until the end of the backup text file.
If you do not have a proper styled backup text file, will say without a full file path, then you have some hard work to do.
You have to re-order the backup text file entries respectively the Mp3tag list view entries to get both sides in sync, then apply the tag-field import.
There might be some other way to go.
Count the lines in the backup text file.
Create a bunch of empty dummy mp3 files, as many as the counted file related entries in the backup text file.
Then do the import of the tag-field values from the backup text file into these dummy mp3 files.
Afterwards load both file sets, the dummy files and the real target files into Mp3tag, then manually copy and paste the tag-field sets from one fitting dummy file to the one fitting real file as it should be (can also be done for selected groups of files).
DD.20120808.1930.CEST