Why won't my truck go into first but goes in all other gears fine?

It feels like it will but when you crank it up and hold it in first it acts like it wants to move then it's grinds. It popped out of first 2 times now it don't wanna go in.

Internal damage! 1st gear/synchronizer failure.. transmission has got to come out to repair! If you keep driving and taking off in 2nd gear,, clutch failure will soon follow!! However, it would be a GOOD IDEA to replace the clutch anyway while trans. is removed, so you decide!!