There are many ways you can say this depending on the exact situation. Here are two suggestions:
- するつもりだった: "I was going to vote, but I lost my voting ticket. I wonder where it is/went!"
- するつもりだ: "I'm planning to vote, but I lost my voting ticket. I wonder where it is!" (or "Where is it?") --> In this case, it might be better to say "but I can't find my voting ticket". This shows that you're still looking for it and/or still hoping to vote. --> Also, "planning" is better because you can't actually vote until you've found your ticket.