Lost data after system restore

Hello. I was recently restoring my system using Dell Support Assist OS Recovery. My drive was partitioned into two drives: C (system files) and D (all the data). During the process Support Assist suggested to backup all the data which seemed to be completed successfully. Yet, after the system was restored I discovered that only files from disk C was restored, while all the data from D was lost. How is it possible? Is there a way to recover the lost files?