yes, this can be done,
it would be a whole lot easier if you had read the tutorials and backed up your C-drive when modding / ftping
ah well,
looks like you will have to read tutorials now!
now, what EXACTLY did you do to the xbox.
the soloution may be as simple as creating an EVOX boot disk and using that to FTP a dashboard back to the xbox (Yes, you can download the original xboxdash files from places)
or you may have to get complicated and use HEX EDITORS, CONFIGMAGIC and other software to sort it out
hint, the words in capitols are good keywords for searches,
I can also recommend reading the tutorials
in summary, YES, its fixable