Not clear whether it's turning brown or IS brown. If its in a pot, the winter will have punished it severely - if there's no green growth at all, scrape back a bit of bark on the main stem to see if its alive or dead - if its alive, it shouldn't look brown and dry under the bark.

i had a similiar problem last year when my bay lost 1/2 of its leaves. in my case I had recently repotted but this was followed by cold winds hence the plant went into shock. i moved it to a sunny sheltered spot and it fully recovered