Hi Paul – how strange! I suspect one of two problems here – the first is that your templates have been edited and an error is either preventing the template from displaying in full or, though not so likely, an amount of code has been deleted.
Have you edited your event calendar templates at all? If so, did you set them up in a sub-directory within your theme directory (as recommended in our Themer’s Guide – link on the right under Useful Resources)?
Secondly, this could be a conflict with another plugin or even your theme. If I were you I’d consider the first possibility before you look at this one, but the basic troubleshooting steps we recommend are to disable all other plugins and switch back to a default, unmodified theme such as Twenty Eleven. If that solves the problem you should reactivate everything one-at-a-time until, hopefully, you discover which component is causing the problem.
Please just let me know if you need any further help here.