Hyundai Repair: 1998 Hyundai Sonata Manual Transmition/Clutch problem, clutch master cylinder, clutch system

Question
I have a 1998 Sonata with manual transmition, and I have never had any problems (80K) with it until now.
The engine runs smoothly, but I am not able to put into any gear when pushing down the clutch pedal. I have checked the fluid level and it is OK, but the color is dark grey.
What can cause this problem? And, what can I do to fix it?
Thank you for your answer!

Answer
This could be a hydraulic problem or a problem with the clutch itself-- something in the clutch could have come apart and gotten stuck in the pressure plate.

Watch the slave cylinder pushrod while someone else operates the clutch.  If it isn't traveling full stroke, bleed the clutch system.  If it still won't travel full stroke, you probably have a defective clutch master cylinder.

If the slave cylinder pushrod does travel full stroke but doesn't release the clutch, then you'll need to remove the transmission and inspect the clutch itself.