You are sewing along, possibly you just started or have been sewing for a couple hours, all of a sudden when you go to start sewing again your machine is angry. Your machine jams or makes large loops on the bottom. I know this can be extremally frustrating and headache inducing, but there may be an easy fix. Take the top and bottom thread completely out of the machine and start from scratch. When threading, pay attention to the take up arm. (The take-up arm is the piece the blue arrow is pointing at).

Below is picture the right and wrong way for this take-up arm to be threaded. If the thread does not go through the take-up arm it is very likely that you machine will jam and not sew (or cause large looping to occur on the bottom side of the material). Once rethreaded, your machine should go back to sewing normal. If not there may be larger issues going on.

So next time your machine is testing your patience by not sewing or making large loops, take a deep breath, take a little break, and completely rethread your machine. This might be enough to get you back up and running.