My 95 nissan hard body 5speed manaul transmison will not go in to gear what is causing this problem?

Does it shift easily with the engine off? Then with engine running, the gears clash? Then either your clutch disc is needing replacement or the clutch pedal hydraulic system needs attention. Look on the transmission bell housing where the slave cylinder mounts (assuming it is hydraulic, and not cable operated). A small rod extends from the slave cylinder when clutch pedal is pressed, and moves the clutch fork a small amount- about an inch or more.

If the slave does not move the clutch fork, check the fluid level in the clutch master cylinder on the firewall. If it is full, you may have a bad clutch master cylinder or a bad slave cylinder. Open the bleed valve on the slave. If fluid streams out when the clutch is pressed, it could still be the master that leaks internally under pressure, but try a different slave cylinder first. It is much cheaper than the master cylinder.

Replace the clutch