If you installed Ubuntu 14.04 with the replace Windows 8.1 option then it replaced Windows 8.1 completely which means that Ubuntu took over the partitions that used to be occupied by Windows 8.1.
Even though the partition has been "deleted" the data should still be there as long as it was not overwritten by new files.
You may still be able to recover some of the data from the Windows 8.1 partition using some of the tools in this documentation. https://help.ubuntu.com/community/DataRecovery
Keep in mind you may not be able to recover all of your files, and some of the files that you do recover may be corrupted.