This method only applies if you're using DeskNow 3.0.x or earlier. Otherwise, see http://www.desknow.com/kb/idx/9/135/article/

1. make sure the user has not recreated the folder with the same name. if so, delete it (you can login as the user, with the su-like login - see administration manual)

2. as a precaution, backup your desknow database

3. restore the database backup with another mysql database name (ex. desknowold)

4. make sure desknowold is accessible by the desknowserver user

5. if you deleted the folder with all the contacts in it (skip this step if you deleted the contacts, but not the folder): in mysql console or query browser, run this query:
insert into desknow.dcontactfolderdatum select * from dcontactfolderdatum where name='nameofcontactfolder' and idowneruser in (select id from users where username='joe' and iddomain in (select id from userdomain where name='mydomain.com'))

6. run this query:
insert into desknow.dcontactdatum select * from dcontactdatum where idparent in (select id from dcontactfolderdatum where name='nameofcontactfolder' and idowneruser in (select id from users where username='joe' and iddomain in (select id from userdomain where name='mydomain.com')))

this syntax has been tested with MySQL 4.1

NB replace nameofcontactfolder, joe, mydomain.com with the proper folder name, username, domain name.