Fabric is not being fed through the machine.
Check to see if the "dogs" (the feeder grips) are engaged. Many times I've used my machine for free hand embroidery or sewing on a button and had to lower the dogs. I've then forgot to raise them and wondered why my machine wasn't feeding through fabric.
Thread bunching under the fabric, weak seams or uneven seams.
When your computer is acting up your first instinct to turn it off and on again. Re threading is the equivalent for a sewing machine.
If re-threading doesn't help double check your tensions. I filmed a quick example of how incorrect tension can effect your sewing. This also means you can finally put a voice to the blog :)
Skipped stitches, fabric getting damaged, thread still snapping despite tensions checked.
When we start sewing the books always say "new job new sewing needle" and yet so many people never actually use a fresh needle. Needles work out at around 50p, a fraction of the cost of the fabric, thread, interfacing, buttons that we use on a sewing project. I've actually served someone in the shop who refused to sew denim because she would have to change her sewing needle, something she had not done since buying her machine 4 years ago!
It is also important to use the correct size needle and the correct type of needle for the task at hand. Your sewing machine manual will have a chart showing you which size and style the recommend for each fabric.