A project manager leading a large, time-sensitive, global IT server replacement project was informed by a server expert on the team that they had a

A project manager leading a large, time-sensitive, global IT server replacement project was informed by a server expert on the team that they had a technical problem that they did not know how to resolve. The team member explained that only one person in the company may know the answer and they refused to helped.This one-of-a-kind expert claimed that the "expert" team member should figure it out for themselves and that he was too busy on his own important work to help.

What's a project manager to do?