Regrettably, such a problem could have many different causes, and it's tough to diagnose them from this far away.

Aside from fairly obvious stuff (look for switches you haven't yet discovered, check for tripped GFIs and breakers) I'm afraid I don't have any suggestion other than to call an electrician. Sorry.

Can you give us more information about these fixtures? Is this new or old construction? If the latter, how old?

As David suggested the first thing to check is tripped breakers or blow fuses. A light switch hidden some where or a GFCI that has been tripped.

If they are the pull chain variety, it's not uncommon to see the switch fail, especially if they are older. There's also a small spring metal tab at the bottom, center of the fixture that makes contact with the center of the light bulb. Over time that tab can loose it's spring and no longer contact the bulb. They can be bent back with care but be darned sure the power is OFF.