Difficulty to say 'No, It is impossible'...

This morning a user wanted to recover à file saved with problems on the networkyesterday at 7:30 PM. She thought having saved it... but the file was nowhere ! We both scanned file server, local drive, temp folders and trash bin... without result.

I had then to say : "No it is impossible to recover your file , you have to do it again"
She Answered : "I was almost sure it was a problem, thanks for helping"

This means I saved a lot of time by saying "NO , I can not"...and of course explaining this answer !

Not only on support, but also regarding projects, I'm only starting on this : being able to say "No".When it is impossible whatever the reason is : technical, out of time, unknow process for me, and then I save time.

What is your experience on this ? Are you able to say "No" ?