Oops! Sorry to hear that. Yep, there is massive change to the data directory on a migration to 2. Data directory now compatible only with 2. And the db has also undergone massive changes.
If instance is a virtualized server, it's possible that server admin has a snapshot (ie, backup) that he/she could access to restore. If remotely hosted, possible the hosting provider has a backup of your site. The former only as good as the snapshot date. The latter could involve some extra cost. And, it's not only the data directory you need but a snapshot of the DB to match.
Other than those, know of no way to reverse it with any 'magic button'.
BTW, the process is a migration ... 1.9.highest should be used ... and one cannot hyperjump from 1.9.highest to 2.6. Must migrate to a 2.2 first, then one (supposedly) can upgrade to 2.6 ... IF server meets PHP, MySQL requirements.
Hope you have at least course backups archived some where.
'spirit of sharing', Ken