Verify that you have the proper file/folder structure in your index directory.
You can find where your index directory is by looking in your ASConfig.cfg file which is located in:
Windows: Program Files\Beginfinite\retain\RetainServer\WEB-INF\cfg
Search for the tag: <indexPath>
Once you are in your index path verify that you do have at least the following:
If you are missing solr.xml, zoo.cfg, or zoo_data/, move whichever files you do have (not including the retaincore directory) into a temp folder and restart Tomcat. The software should automatically create the necessary files/folders after Tomcat loads.