From Wikipedia:
In October 2011, Douglas Gresham (C. S. Lewis' stepson) stated that Walden Media's contract with the C. S. Lewis estate had expired, with Walden Media no longer has exclusive purchasing rights to any further Narnia films, thus any production of a future film was on hold indefinitely.
It was originally assumed that 2014 is the earliest that production on another Narnia film could begin, as that would be the earliest the moratorium placed on the C. S. Lewis estate's right to sell the film option of the series will be lifted. However, in May 2012, Gresham confirmed that technically any studio still has the option of making Narnia films during the moratorium, but without the involvement of Walden Studios it cannot be released until 2018 at the earliest (the actual end year of the moratorium).
Gresham also hinted that Walden Media's lapse in renegotiating their contract with the C. S. Lewis estate was due to internal conflicts between both companies about the direction of future films.
Contrary to Walden Media's initial plan, Gresham stated that he plans for The Silver Chair to be the next film to be made, hinting that future films might be made independently.
