Can you post pictures of the screen settings? What setting are you on when this happens?
Do you have a preferences page on the screen of this machine? ( I don't know the model specifics.) Check to make sure zig zag is not the default stitch go through all the settings to make sure they are on default/original settings.
Do you have a picture of a lock on the home screen? That button is used to lock the stitch selection so you don't accidentally hit something on the screen and change the setting.
Go to the manual and these will be explained in more detail if the machine has them.