From the FAQ's, under "what if I notice an officially closed record etc...."
You will need to conduct a search for that person on the site to find the record. If you cannot find a person you believe is recorded in the Register, the record may be officially closed.
You need to prove that individuals listed in the records are deceased in order to submit your request to us and for the record(s) to be opened. We can only accept a copy of an official death certificate (issued by the General Register Office or overseas equivalent) so please ensure you have this documentation before proceeding. The only suitable form of evidence is an official death certificate (files supported: .jpg, .png, .gif). If we do not receive this valid proof of death, we will be unable to proceed with a search.
You can find more information about providing proof of death here.
Additional information can be provided such as siblings, occupation, evacuee or any other relevant information that may help us to search the data.
have you tried to find her under a married name?