Did the file convert but you can't enter a password that it will accept when you try to open it?
Check for differences in capitalization in your password. Older versions were not case-sensitive. Newer versions such as filemaker 11 are case sensitive.
You may be able to open your original file with the older version of filemaker and see the password to check on this.
What version of filemaker are you converting?
Yes. That is exactly the experience I was having. I was able to open the old file and check the capitalization. I was trying to convert from filemaker 6 to 11.
I have 5 different database that were relational. They seem to have no relation now. Is there a trick to making one database relational to another?
Relationships between files are based on filenames and possibly the location of the various files within directories. This is true for both your old and new files.
If you convert all the files in a batch without moving any files, filemaker should update these links so that they still work after conversion.
If they don't, you can open Manage | External Datasources... and update the links there. This is something you'll want to do anyway as old filemaker files often have a nightmare tangle of layered external data source references that need to be cleaned up anyways after conversion.
You might also want to read this article on file conversion: