Run time error 1004

I am working on Excel File and VBA. Can anyone tell me what's wrong with following command:

Private Sub optMonday_Click()
Sheet1.Unprotect
'Now here I want to enter some values.....and in the next line want to re-protect sheet....
Sheet1.Protect
End Sub

Tha above always displays an error as:

Method 'Unprotect' of object '_Worksheet' failed

similarly, Method 'Protect' of object '_Worksheet' failed 'when I try to Protect it......

Please let me know, how I can Protect and Unprotect the Sheet dynamically...

Thanx in advance.