You did try changing the bulbs, right? <If the 48" has 2 CF
bulbs, you might need to change both bulbs. Some lights are wired in series & if one bulb is bad, both won't light.>
It is most likely a connection problem. you need to check all the wires/connections with an ohm-meter. <I am sure you know to unplug the lamp first before you take it apart .... Fluorescent ballasts put out HIGH voltages!> Also check that you have power going to the lamp - plug something else in the socket to make sure it works.
I don't know how to test a ballast short of swapping it with another one. <I've never have a ballast failure & I have fluorescent fixtures all over the house, some going back 20+ years.>