When this happens you should start by checking the local status page of the switch (es) that are offline. This will tell you why the switch is offline.
Some causes I have seen; partway through a firmware upgrade (you think because the others are done this one should also be done - you just need to wait longer); stuck doing a firmware upgrade (hold the reset button in for 10s); lost connectivity to the Internet for some reason, perhaps upgrading another switch in the stack has caused this stack member to get cut off from DNS or the Internet (sometimes you might need to just reset the switch, other times you might need to pull it out of the stack, manually connect it to the Internet somehow so it can complete the upgrade, and then put it back into the stack).
The number #1 issue - not waiting long enough. Thinking because something else has finished it should also have finished. I have personally been caught by this more times than I would like to admit.