Filemaker saves data automatically. Thus the contact records won't just vanish unless something has taken place to delete them.
Are you sure you've opened the same copy of the file as previously?
Did you delete any records in any table in your database after entering these? (Cascading deletion of related records can take place if that option is selected in the relationship graph.)
Any scripts that run when the file is opened, closed or in response to a script trigger that deletes records?
Do you have a script that uses go to related records to pull up a group of records and delete them? (GTRR in a script can have unexpected results if there are no matching records and the script doesn't test for this error.)