A couple days ago my garage door went up and then wouldn't go down. Neither the clicker nor the button on the wall will make it operate. It has motion sensors. The unit is getting power because its lights are on. When I examined it, the Learn light is flashing. I followed the instructions to reset the buttons and to program it and it wouldn't operate.

There was a lot of rain that day but the unit is dry. The spring works. When you click the opener or wall button, you hear a clicking noise and the light flashes, but that's it. There is also a "Lock" button on the wall pad. The unit is a 1/2 Loadmaster unit.

Any thoughts? Fix it or is it dead?

Insure that it is not "Locked", insure that nothing is obstructing safety light beam across the door (which I assume is what you mean by motion sensor). Also make sure that receiver half of safety light sees light beam.

Reread your manual regarding exiting the learn mode. Do you have a built in button in your car, does it work? I did one of these installs last week and some parts were like working with a old time computer, very stubburn.

We have tried to things suggested including unplugging system for 20 seconds, etc. We noticed that on the sensors, one is green and one is yellow. Could this be the culprit? How do we correct?

It's a Chamberlain Liftmaster 1/2 HP. Their home page says the manual is not ordered.

One green, one yellow just means one sends and the other receives, that's normal for Chamberlain openers. What does this mean, "Their home page says the manual is not ordered." Do you have two buttons one purle and the other black or just the one button?

Sorry, meant to say it says that the model is not found. So I can't find the exact model instruction manual. Its only from 2004, so I can't imagine why its not there.

When I click the opener (its not attached directly to my car) or the wall button, the light comes on and I hear a clicking sound, but no motor movement...

You may have a bad board in the "computer", I'd call Chamberlains help desk and ask them to walk you through some troublshooting.

According to the User Manual (that I found online) I have to replace the logic board. Is this a difficult thing to do?

Well no, not if you have ever installed an external hard drive on a computer or installed a fax/printer/copier on a computer. That board will be fairly pricey considering an entirely new Chamberlain can be as low as $150. 4-5 years seems too short of a life but I would not spend even $50 to fix this unit and you may be wrong and just installed an unneeded part.

So you say replace rather than repair? Thanks for your help on this...

Yep, repair of a computer board is beyond nearly everyone. I have to replace a mother board in a thread mill, it took about 30 minutes and 1 small phillips screw driver.