if the lights are in series, (probably, if they are controlled by the same switch), all it would take would be one loose wire on a switch, or at a fixture to cause them all to not work. Probably easy to fix once you locate it, but sounds like there are quite a few possible locations to look at. The most likely place is at a light fixture if they were installed using wire nuts. It's not uncommon for one wire it come lose due to heat/cool cycles when the light is on and off, or due to vibration from people walking on the floor above. Shouldn't happen, but it does.
If you're handy, turn off the power and take off each fixture one at a time, and make sure each still has two wires attached, then do the switches. As long as the power is off, it's safe to touch, and usually only requires a screw driver.
Good Luck!