Number of posts limited to 100

My site seems to have capped the number of posts it displays to 100. I have all my posts in a calendar and once a new one is posted it removes the oldest post from the list.

In the back end the original posts are still shown and you can access the post using the url, they are just no longer searchable or showing in the diary entries/calendar. For example all of February and March 12 have now disappeared from view.

I’m pretty sure this is tweakable by editing themes but having gone through the page throughly I can’t spot any code that relates to total number of posts visible.

Is there a plug-in that increase this?

Thanks