I hope mamjet's reply is facetious - but just in case 'will readings' only occur in Agatha Christie novels. In real life they don't happen, particularly for non-millionaires..
The advice about the Probate Office is good, but if your question is about not being able to find the will in the first place then that can be difficult. Search her house, and ask any banks that she may have had an account with and any solicitors she may have had dealings with. Ultimately you may have to assume it just doesn't exist.