I have a track light of three mini-floods. (not sure what brand.) I have replaced bulbs before. Now, one end light (never been replaced) is not working. I put a new bulb in, and it still won't work. I have made sure that the connection is still there, and that the individual on/off switch on the lamp itself is "on". (I toggeled it, just to be sure). Any ideas of what to do next? The other two lights work just fine in it.

Look at the contact points of the fixture for bent, broke or burnt.
Then correct.
Also, be sure that if fixture was installed 180 Degrees out, it will not work.
New bulbs have been known to be bad, use a known good buld and or try that bulb in another fixture.

Outlet puts out 90 volts. Known good 60watt bulb won't light up. Why?

Reattach the fixture, it may need to be installed 180 dgrees from the way it is installed, Be sure the side with 2 contacts of the track is connected to the fixtures 2 contacts on the side.

90 volts?? Why not 120 volts?