Find contacts on CiviCRM
(Adding this answer based on Comment by Myriam)

This could be a Permission issue. If the contact is in the db but you do not have permission to see them then you won't see any results.

Ways to confirm this would be use the QuickSearch and just hit ENTER (or type % and hit Enter if the former has no effect), do you see all the results of contacts, ie your complete list. 

If you do and you still do not find them, then you may also need to search in Trash

If you only see a subset of your contacts then there could be some ACL that is restricting which contacts you can see

If you only see your record, then it is likely to be civicrm Permissions which are set by your CMS