I have a Miracle One gate opener and, as I've done a hundred times previously without incident, I clicked remote to leave gate partially open (to take trash cans out), about 20 seconds later, the gate then opened all the way on it's on and will now not close. I tried unlocking the master and slave arms with the key and neither side will close manually (can they be stuck?). I opened the control panel and "strike open" and "timer" LEDs are rapidly flashing. All the other other lights appear to be okay? Is this something I should try to tackle on my own or should I call a repairman?

As a gate automation technician I fix a couple of these every month.
There are a dozen reasons the Miracle may not be closing. These include the following:
Something may be telling the gate to stay open such as vehicle detectors, photo eyes, open switches, stuck receivers and call boxes locked on open.
It might also be a low battery system. This opens the gate when power is too low and holds it there so you don't get stuck. Commonly it is because the batteries are old. The manufacturer recommends changing them at three years.
There also might not be enough power to charge the batteries. Check for a bad transformer, breaker or ground fault interrupter (GFI) circuit. The GFI can be located either at the transformer or if the circuit is not dedicated, then elsewhere on another outlet.

The likely reasons you may not be able to manually close the gate is either the key release is not open all the way or the big drive screw has not been lubed in a long time... or both.
Remove the cover via the four screws and use penetrating oil on the screw. First lubricate the lock at every point. Then wiggle the gate back and forth (make sure the gate is in the unlocked position) until the gate begins to move freely. Once it is moving freely, the gate screw should be lubed with a light grease.
There are a series of steps to checking the battery but if all the other steps fail, you should call a technician before going further. There could be serious and expensive consequences to replacing the battery and other components without experience specific to these operators.

The memory could have been somehow corrupted. I would suggest that you call in an experienced person to reset and reprogram the open/close positions.
Hope this helps :)