This method only applies if you're using DeskNow 3.0.x or earlier. Otherwise, see http://www.desknow.com/kb/idx/9/134/article/

1. if the original calendar was deleted (if the user only deleted the tasks but not the calendar, skip this step): make sure the user has not recreated the calendar 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 the original calendar was deleted (if the user only deleted the tasks but not the calendar, skip this step): in mysql console or query browser, run this query:
insert into desknow.dcalendardatum select * from dcalendardatum where name='nameofccalendar' 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.dcalendareventdatum select * from dcalendareventdatum where etype=2 and idparent in (select id from dcalendareventdatum where name='nameofcalendar' 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 nameofcalendar, joe, mydomain.com with the proper folder name, username, domain name.