prezios wrote:I can no longer put my Volvo 340 automatic in gear. When I press down the knob on the gear lever, it just starts "grinding"
What could it be?

Earth issue on the lever not allowing the de-clutching servo to work (you should be able to change gears at idle with out pushing the button).
First make sure the de-clutching servo clicks when the button on the gear lever is pressed.
If it does not, then earth or mircoswith.
If it does, check that there is vacuum at the pipes.
If there is vacuum and it clicks, then check the clutch shim gap, as said you should be able to change gears with out the clutch, the clutch engages at about 1200rpm, so make sure your idle is less than that.
Hope that helps for a start.
Ben