Talk to your agent about your concerns. If you decide (or if you've already decided) you want to terminate the agreement, tell your agent that. If he/she is reluctant to do so, ask to speak to the agent's managing broker.
Incidentally, I believe a 6 month contract is WAY too long. Next time, go with a 3 month contract. Tell whatever agent is looking for your business that if you're happy with their activity--even if the house hasn't sold yet--you'll be glad to re-list.
If you concern simply is that the property hasn't sold--but you're overall happy with your agent--ask your agent to run a new CMA to determine market value. As the others say, if you haven't seen any action on the property, most likely it's overpriced.
Hope that helps.