Will not go into any gear with the truck running

Hey guys, yesterday on my way home from work when I pulled away from a stop sign and went to shift it would not go into gear. Will not go into any gear with the truck running, with the truck off it goes into all gears fine. I was able to get the truck home shifting without the clutch, no noise or grinding going on. Last night I replaced the clutch slave cylinder but still the same problem. Truck has 175k miles on it, I bought the truck used so don't know anything about the clutch so I think I need a new one can you help. Thanks