Clutch engages but, gears do not. what causes this? How expensive is this to fix

With it running , does it shift through the gear positions as it used to , or is there something else missing , in the 'details'? Like - was the clutch just replaced or does this thing have 200K or more on it?

If your sure the clutch engages but no power is transmitted through the transmission in any gear, you have an unusual trans problem. I recommend have your clutch engagement rechecked by a pro. Or maybe you have a rear end problem.