Facets and Apache Solr in Drupal 7 - Clicking Facets results in no results being show
This is very odd behaviour, can you ensure you are:


running the latest version of apachesolr (3.6.1)
running the latest version of the apachesolr module (7.x-1.1) OR search_api module (7.x-1.3)
are running the latest schema.xml and solrconfig.xml (hopefully the 3.x version)


Also, try dropping and re-creating the entire index, this normally addresses more problems.