2005 Hyundai wont go in gear when i hit the clutch

If manual
collapsed pressure plate
worn clutch
hydraulic master or slave cylinder leaking
syncro internal transmission issue

if auto
low oil
torque converter collapse
gearbox solenoids or electronic fault
internal mechanical damage

Your gearbox is still turning with the clutch depressed, which means it has not released properly.

The most likely cause is low fluid and air in the clutch hydraulic line, which then needs to be bled, or if a cable operated clutch, the cable is stretched or broken, and needs replacement.

If it happened very suddenly the likely cause is that the clutch throwout fork, inside the bellhousing, is bent or broken, and cannot push the release fingers down.