I’m sorry to see you’re having problems and can indeed see this on your site.
It actually looks like the Javascript which powers the calendar widget navigation is not being loaded: to find out why this might be, can you deactivate all other plugins and switch to a default theme?
Does the problem still occur?
If that solves the problem, please reactivate each plugin and finally your theme – testing at each point to check if the problem has returned (this could be helpful in isolating the source of the problem).
Let me know how it goes!