Has your sewing machine "crashed?" Is it not sewing evenly? Is the thread breaking? Before you take it off to the shop for an average minimum of a $60 shop fee, there are many things you can fix it yourself. Most sewing machines are very sturdy, and simply need to be tweaked or properly cleaned to have them working properly again.
First of all, if the problem seems to be that it won't work at all (worst case scenario), check to be sure cords are secure in both power outlet, and where it enters the machine. Also, is it plugged into a power strip (make sure the switch is on) or is there a wall switch in the "off" position? Has the machine been turned on? These scenarios sound silly, but they do happen. If you still hear no signs of life in the engine after checking all of these areas, you may have to break down and take the machine to a specialist to be repaired. However, if the machine comes to life, then there are other things you can try.
The most frequently found problems are those related to stitching. The stitches may pull, thread seem to be stuck somewhere, or stitching may be uneven. The first thing to check is how the bobbin is inserted, placing in the machine as directed by the instructions. If the thread faces another way, you may have problems. When positioning the thread on to the machine through to the needle, the same holds. If you miss threading through one loop or hook, it will not sew properly. This can move as you sew sometimes as well, so even if it started out in the correct position, this may have changed. Also, the needle you are using must be appropriate for the thread. For example, if you are using thicker thread in a fine needle, chances are the thread will break. And if you are sewing on material that is of fine texture, you cannot use a coarse needle and thread, but must use a thinner needle and finer thread. All of this will affect the way the machine will --- or will not --- sew, as is often the case. Your machine has two thread tension adjustments on it that also determine whether it will sew well or not. One is found at the top of the upper thread guide, and the other usually within the part of the case that opens for changing of bulbs. Be sure to consult your manual to be sure these are set correctly, and you can adjust slightly until the problem is solved. There is also a tension screw at the bobbin case, and this too, needs to be set properly. All of these things can cause problems for your machine.
Always keep your machine clean. Your machine will have come with a small brush. Use this to help clean out the bobbin case (take off the cover and remove the bobbin), and also under the area of the bulb cover. If you have a computer brush attachment for your vacuum, this works great for cleaning your machine also. Check the bobbin area and all loops and grooves where the thread is located. Be sure nothing has gotten stuck and remained in the machine. Stray pieces of thread can really interfere with the operation of the machine. Also, check the feed dog and area around it. Disengage the feed dog to do this. Lastly, oil your machine regularly. Refer to your manual for where to do this, and how often it is needed for your particular machine.