Best strategy to restoring backup by importing csv file
In general, importing/exporting using csv means you will lose data, eg log history of change, the "real" creation date instead of the restore one... 
If you can at all, I would suggest to backup restore the database itself, rather than only the contact via csv.

This being said: do you have an error message? it might be a matter of memory, try to increase what php can use. Or select on the export what fields you really want to export, the default tend to include lots you don't really need.

If you can, try to create a temporary table that you import/export with phpmy admin (not civicrm_contact, say got for backup_contact), on the import step you can choose to import from sql rather than csv