Cut your losses and chunk the Craftsman garage door opener. Get a Genie and you will never post a question about openers on this forum again. A little research on this forum will show you hundreds of posts about those badly engineered and cheaply produced monsters. There are probably more questions about them than anything else on this forum. I recommend the Genie "Excelerator". Opens twice as fast, quiet, has two lights, and extremely reliable. See the Genie link below. Here is a link that might be useful: Genie Excelerator at Home Depot

Thanks Don, I should've mentioned the up force was at 9. I found and called the cust svc number 800-528-5236, and they made me lower the up force to 5, manually lift the door waist high and drop it. Since it slammed to the ground, their conclusion is that the door is not balanced and thus the GDO is doing all the work. they recommended a couple of repair guys to do what's called a Door Balancing. Does this make sense to you? Thanks again.

If it is stuck in the open position you will most likely find the trolley is jammed against the bolt just in front of the motor unit. You will need to loosen the chain enough to release the tension and then run the opener until the door closes. While the door is closed locate the 'up limit' on the side of the motor unit and turn this control 2 full turns in the opposite direction of the arrow printed on the cover. Tighten the chain.

Random flashing of that LED is normal but it can sometimes send a signal when there is a problem. It will flash a number of times, pause and then repeat. It sounds like you may possibly have two problems involved here. On the rear of the motor unit you will find a control marked 'down force.' Increase this just a bit. On the side of the motor unit you will find a control marked 'down limit.' Turn this a bit in the opposite direction of the arrow printed on the cover. Some tweaking will be necessary. Both of these problems can be intermittent.