First choose your Croft. There are three in England within the counties of Yorkshire, Cheshire (which in old document may show as Lancashire because of boundary changes), and Leicestershire.
Within the Croft (aka Croft-on-Tees), Yorkshire, Margaret Prest appears in the 1861 census as a patient, born in Croft 1836, in what appears to be Croft Spa. There may have been a hospital but I found no record of a hospital within the time available to me.
familysearch.org gave no clues to any possible siblings, marriage or parents to Margaret, and I have my doubts that she was born in Croft. It's possible that the census returning officer entered Croft as a matter of expediency since he lacked the true birthplace.
She could have been born in County Durham which is joined to Croft by a bridge over the river Tees.
I can only add that with the lack of any other information this is likely to prove a difficult search. You could start with the parish records for Croft which are available at The North Yorkshire Records Office :- http://www.northyorks.gov.uk/archives
. They have parish records from 1615 to 1994. Burial records might be good between 1861 and say 1863 if she was at the spa because of ill health. Generally a burial record will give a fathers name. Croft Spa records could be equally useful if you can locate these because they would probably say who was paying the bills.