Did you make sure that the thread in the bobbin is going the correct direction? Have you cleaned the machine? Do you have the needle positioned correctly, do you have a new needle, the correct needle for machine and fabric? Are you making sure the bobbin case is clicked in securely? Have you tried to change the thread to something different? Can you remember what you did before the machine starting not sewing correctly? Is the thread from the top spool coming off the spool in the correction direction?
There are several tutorials about sewing maintenance. There is a series of videos about sewing maintenance on You Tube that is very helpful and educational, just type in the search area at their site.
Good luck with getting your machine back to sewing nicely for you. Have you had your machine checked out by your local sewing machine service repairman? Maybe it's time to have her professionally serviced.
Pam M