Did you switch to using LED bulbs? If heard weird things can go on sometimes. Some technical reasons, didn't understand it though.

The voltage "disappears" because your meter is reading junk. Its impedance is so high that it can read "phantom voltage," even when the circuit's resistance is so high that no usable current can flow. When you put a real load such as a light bulb on the circuit, the truth is revealed. :)

A low impedance tester (example) will give you more realistic results.

For a cheaper low impedance tester, get a rubber pigtail socket and a small (7-25 watt) INCANDESCENT bulb at the hardware store. If you have a soldering iron or gun, tin (coat with solder) the wire ends to stiffen them so they're more like tester probes.

In your case, the circuit is open somewhere. A failed switch is a strong possibility. Use your low impedance tester to check both switch terminals (put the white wire on the metal box, or on a bare ground wire in the box, and touch the black wire to each switch terminal in turn). Careful when working with live circuits! With the switch on, both terminals should light the bulb. If not, replace the switch.

If you get real voltage at both switch terminals, but not at the black wire in the light's box, post back and we'll dig deeper.

The link is for illustration, and is not an endorsement of the product or vendor.