transmission problem

This may be obvious to some but here goes. My 1999 tahoe 4l60e transmission with 190 000mi won't go into 3rd gear only revs. No loud noise no burnt oil smell just dropped out of gear while driving. I was reading about this problem and just making sure, is this a major transmission job or a minor one (relatively speaking). To rebuild or not is the question. thanks.

The unit must come out and come apart, so in effect it is basically a rebuild.

can someone also maybe explain the problem inside the trans itself. Is it the clutch? thanks

3-4 clutch pak could be the problem, again without disassembly there is no sure diagnosis.