If you have recently created a new multidev environment on Pantheon and search isn't working for you, this is because you will need to repost the schema and rebuild the search index to get it working.
In multidev, Pantheon is creating a new, full-stack environment for your site, including a new Solr instance but it does not provision a search core for you until you post the schema.
When you login to your environment, go to:
https://SITENAME/admin/config/search/pantheon
and you should see the following:
"The Pantheon Apache Solr service in the SITENAME environment was not found, have you posted the schema in this environment yet?"
Follow the prompt and go to the post schema tab
https://SITENAME/admin/config/search/pantheon/schema
and post the recommended schema -- it will then show that it has connected and provisioned a new schema for your site, ex:
Core and Schema from Solr Server
Core
f63864b1-bfdc-4396-aac2-63761eec5fdc-SITENAME
Schema
drupal-4.3-solr-3.x
Search still won't be working for you until you rebuild the index. Go to the Search API page:
https://SITENAME/admin/config/search/search_api
and click on your node index. From here, it is recommended that you take the following actions, in order:
Clear all indexed data
Queue all items for reindexing
and finally, Index all items now