There are a few things that can cause this, some already mentioned.
First, remove the fabric you're trying to sew and get some scraps of it. Use the scraps to test things so you don't damage what you're trying to sew.
Feed dogs--those little jagged things that stick up from slots in the flat plate under the needle--need to move when the machine runs. Hold the ends of the upper thread and the bobbin thread, the use the foot pedal or the hand wheel to make the needle go up and down. If the feed dogs move, that's not the problem.
Bobbin threading is a real common issue. Either the bobbin is inserted incorrectly in the first place--reeling off from the wrong direction, or not properly threaded through its case--or the bobbin tension is messed up. When you try to sew, does the thread on the underside of what you're sewing wad up in knots of way too much thread? Then the issue is the bobbin, and once it's sufficiently snarled up, the feed dogs don't work. It's your machine's way of telling you, I guess. Note: I strongly recommend against adjusting bobbin tension on your own. It's really finicky and hard to get right even with much experience.
Threads on top and in the bobbin are mismatched so much they can't play nicely together. This can happen if one is substantially finer than the other. Often it'll let you sew for an inch or less, then the feed dogs stop working.
Foot pressure is not sufficient. Check the manual for how to increase it--but know that I have never once had to change mine, whether I'm sewing three layers of denim or hemming chiffon. Maybe I've lucked out on my machine not getting out of adjustment.
The stitch length has been shortened or set to zero. How much fabric is pushed through by the feed dogs is determined by stitch length setting.
The reverse stitch/backstitch lever or setting is engaged. You want the fabric to go forward, it wants it to go backward. Stalemate!