Check the mic gain.
Make sure it is not in PA mode.
Make sure the mic jack in the radio doesn't have a broke or loose wire.

Those are a few things I can think of based on the information given. More information is needed if these will no solve the problem.

Does it turn on?
Does it key up?
If it keys up is there carrier?
How many watts is the carrier?
Are there any mods done to the radio?
Does it have a roger beep installed? What kind?
Does it have an echo board install? What kind?
Did you do the mod or some one else?
What is your SWR?
Is the display flashing when you key it?
Are the displays lit at all?
Did you disconnect the power cord after you turned it off, and reconnect it?
What channel are you keying up on.
Was/is the case off the radio when you tried it or were you working on it at the time of the failure?
What mics have you tried.
are you using talk back?
did you some one on another radio tell you they do not hear you, or are you going by no swing on the meter in the radio?
Could the meter be bad?