You can increase the List View Threshold beyond the 5,000 default, but it is highly recommended that you don't, as it has performance implications. The recommended fix is to add an index to the field or fields used in the query (usually the ID field for a list or the Title field for a library).
When there is an index, that is used to retrieve the item(s); when there is no index the whole list is opened for a scan (and therefore hits the threshold). You create the index on the List (or Library) settings page.
This article is a good overview: http://office.microsoft.com/en-us/sharepoint-foundation-help/manage-lists-and-libraries-with-many-items-HA010377496.aspx