Hi Hannah,
Ok there are 2 places I would start:
1. Go to SharePoint Central admin > Application Management > Manage the search service and check what that says - are there any errors?
2. How many servers have you got in the farm and what are there roles? Central admin > Operations > Services on server
Hopefully that information should give us a better idea of what you have an where exactly the issue lies.
Jenny
http://www.sharepointblogs.com/jennyeverett