Bckup failed

I have created a Maintenance plan that was taking backup for years now. It failed last night with following error. What does it mean?

 Executing query "DECLARE @Guid UNIQUEIDENTIFIER      
EXECUTE msdb..sp".: 100% complete  End Progress  Progress: 2012-11-29 00:00:01.79     
Source: Back Up Database (Full)      
Executing query "EXECUTE master.dbo.xp_create_subdir N'F:\MSSQL\BAC".: 33% complete  End Progress  Progress: 2012-11-29 00:00:01.79     
Source: Back Up Database (Full)      Executing query "EXECUTE master.dbo.xp_create_subdir N'F:\MSSQL\BAC".: 66% complete  End Progress  
Progress: 2012-11-29 00:00:01.79     Source: Back Up Database (Full)      Executing query "EXECUTE master.dbo.xp_create_subdir N'F:\MSSQL\BAC".: 100% complete  
End Progress  Progress: 2012-11-29 00:01:36.76     Source: Back Up Database (Full)      Executing query "BACKUP DATABASE [...  The package execution fa...  The step failed