may have to replace both if both are bad or you may have replaced the remaining good one...!!
Put the old one back in and replace the other most likely defective bulb.
otherwise if it a really old flourescent light fixture it may have a defective starter. the starter is a silver cylinder the size of a thread spool on older models if it is defective ,,the bulb will not start up but may try . you can usually see the starter sticking out at one end of the fixture...rotate it counter clockwise to remove and reverse to install a new one. good luck LJ