Something went wrong in the planting process or soon after that. If the sod is laid on prepared soil and pressed or rolled down to get good sod/soil contact, and if it is watered in well and kept moist that should not happen. If the sod dried out before, during or after installation it may be that the remaining green sprigs are going to have to regrow and fill it in again over time. This won't happen until about April when the weather warms again.
One other possibility is that a fungus such as Brown Patch has affected the new sod. This would be evidenced by dry grass blades but green runners now that the damaged areas have had time to dry out. There is not anything for you to do at this time. Except wait. When the weather warms a bit the runners will produce new leaves.
Q&A Library Searching Tips