Garage door opener slips
A well maintained door is easy to open and close.  We use power openers so we can open the door without getting out of the car.  The problem is, this denies us "tactile feedback" of a developing door problem and we don't know until the door goes BOOM.  Assuming this is a broken spring, it would have suddenly failed in any case, but other problems may have contributed. 

Doors are dangerous because they are in tension: the weight of the door is counterbalanced by a wound up spring.  The force of either one can kill you.  

Doors are DIY repairable, you just really, really need to do your homework on the safety stuff.  

Your first step is put the door all the way down, so it's at rest, thoroughly inspect it.  Now when they initially installed the spring, before they tensioned it, a guy shot a line of spray paint down the spring.  With the door down, it should look like a candy-cane with typically 7.5 loops.   If you only see 1 or 2, that's either broken or has come loose from the shaft.  

Then disconnect the opener. If you don't see anything mechanically wrong, then try operating the door by hand and look for anything binding or sticking.  It takes about 20 pounds of force to start lifting a sticky old 12x14 industrial door, so yours should be a feather.