Once you are clear that the only problem is a slightly loose chain then you can begin fixing the problem.
Garage door chain tight one side loose other chamberlain.
If in doubt replace the chain completely.
Make sure that the garage door is closed and that you have another way of getting out of the garage.
Tightening your garage door chain is a simple matter of turning the right nut.
This will cause premature wear on the chain rollers.
Step 2 turning off the power.
Adjust it tight enough so that the center of the chain sags about 1 2 down from parallel with the track graphic below right.
You loosen the locking inner nut and turn the outer nut clockwise to adjust the excess slack from the chain.
Do not over tighten the chain.