try replacing the clutch or check your hydraulic cluch fluid. the fluid tank is located on the firewall, lefthand side.
Is the car auto or manual? check fluidlevels first, have you noticed any burning smells prior to problem? Does it not engage in all gears including reverse?
A throw out bearing is the bearing in a manual transmission that helps to engage and to disengage the clutch.
you either got air in the hydrolic clutch or the hydrolic clyinder is out .check your fluid level.
I have had a similar problem with my 1991 Wrangler. I found if you can pull the 4X4 stick past the 4 high the push it back up it may engage. That worked for me. The problem was ultimately fixed when I had the transmission rebuilt.
GET THE COMPUTER CHIP CHECKED FIRST!!! I had the same problem, for 1st and 2nd gears. I spent $1300. on a new transmission, only to find out it was a computer chip that was bad -$80 problem!!! Get a 2nd, 3rd, 4th opinion at different garages so they don't try to cheat you!!
Most cars this is not a problem, the engines are identical.
If second is the only gear that does not work, there is an internal failure. The transmission will need repaired or replaced.
First you put the vehicle on Neutral then swithc to 4LO it will engage. If a wheel doesn't engage, then you have a vaccum line problem. See owners manual.
Manual transmission: Clutch Automatic transmission: possibly low transmission fluid
clutch is slipping.
Your throw out bearing is the problem.
You will need to specificate your problem, is it a manual or automatic gear box