Is it sectional, or is it up and over as it can't be both. Sectional will run in tracks as it opens, up & over will have vertical runners either side.
I presume the handle is T-shaped and in the middle of the door. Turning it will pull a wire which is connected to the locking mechanism (typically a pin) at the top. From the outside you may have a piece of strip across the top of the door, pull that off and you will be able to manually depress the pin to open the door. You can replace the snapped cable with a gear or brake inner.
Mine did this last month and it's annoying, but quick to fix.