It is possible that you have somehow managed to bend the needle. The bend may be so slight that it is not immmediately visible to the naked eye particulary if the angle of the bend does not allow you to see it in profile. I would try replacing the needle.
Your stitch length is most likley too short. I would suggest taking the bobbin out, brushing out the dust then rethreading the machine and checking the stitch length.
Double check that you have everything threaded correctly, and that the tension has not been changed to an extreme number. If this doesn't work that out the shuttle (holds the bobbin) check to see if there are any scrathes or nicks on it.
I have found that a few times when this happened, the bobbin was not in the machine correctly, but most of the time I found it was because the needle had been bent some way and was not working correctly.
Something on your sewing machince got bumped and it knocked it out of time so that the bobbin is turning when it shouldn't be you can try to fix it your self by re adjusting the tesions on the machine but most of the time you will end up taking it in and getting it fixed at the shop.