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declare @lastbackup datetime
select @lastbackup = MAX(backup_finish_date) from sys.databases db left outer join msdb..backupset b
on db.name = b.database_name
where db.name = 'YourDBName'
select @lastbackup
IF @lastbackup < DATEADD(hour,-24,GETDATE()
BEGIN
backup database YourDBName.....
END