You can be honest here. If you have found this to be a true statement, explain the circumstances and why you felt that way. It is likely that, at some point, you may feel unappreciated, so the interviewer wants to know how you will respond to feelings like that.
"I worked as a volunteer when there was a flood in my hometown. My responsibilities included dispatching volunteers to different neighborhoods. I spent long hours for the first week making sure everyone was cared for. There were moments where I didn't feel appreciated for the long hours put in, but I knew the value of my work and how many people I was helping. So, I kept on doing my thing!"
Entry Level answer example
"I don't necessarily look for appreciation so I would say that I have never noticed any blatant un-appreciation in past volunteer work. I do what I do for others. Not for the recognition."
Experienced answer example
"I think a feeling of unappreciation can be relative. I stick to my tasks, I listen to the needs of my organization, and I help those who need it. Volunteering isn't about me - it's about my community."