No, the box will probably be closed if no one can get there. I know when it snowed last here in the west a class 70 was used as a staff taxi, getting people to/from work when the roads where to bad. If there are other reasons why you can't get in, e.g. Sickness, family emergency, etc.. Other staff are called to see if they are available to cover or spare relief staff are dispatched. Sometimes boxes that can be switched out are switched out and staff from that box covers the gap, subject to signing the box.