Will not straight stitch after doing another type of stitch

Stitch width set to zero, turn off any decorative stitch selections, stitch length to ~2 or 2.5.

Is your bobbin threaded correctly? You should be able to pull out the thread slowly while feeling it very slightly holding back. It should not pull out easily or quickly.
Could be something wrong with your tensions, try adjusting the upper tension first by starting at the middle setting and working gradually higher and then lower till you get a neat even stitch.

Stitch length @ 2.5mm (between 2 and 3 on the scale).
Stitch width @ zero.
Top selector lever @ 1 / 2 or (1 slash 2).
Needle position @ center.
That's it, no rocket science here. Stitch length will vary per application and per user choice. The 2.5 setting is only a suggested default.

The "dial" in the back is for decorative stitches which require a "programmer" to be inserted with the stitch formation you want to form. For a straight stitch no cam is required in the back of the machine. The three dials on the front:

Upper Left: Stitch selector
Upper Right: Stitch lenght
Lower: Stitch width

Using the upper left dial select the straight stitch picture
Using the upper right, choose your stitch lenght
Using the lower, choose your needle position, left/center/right

Have you selected the proper stitch. Look at the decorative stitch line 1,2,3,or 4, then the number above the stitch. Your screen should say 1:24 for line one, stitch 24. Make sure you are using the recommended foot. Embroidery unit is disengaged. Then makes sure you have selected the proper weave or thickness of material button. If it still won't sew, try a different type of fabric button and try that. This will change the tension. The tension could be too tight or too loose for that stitch.

You obviously have a faulty machine take it back to your dealer where you bought or directly contact Bernina.
Best wishes.

Hi Susie,

I can't tell whether you mean straight stitching and then turning your work 180 degrees to stitch atop the straight stitches in zig-zag, or continuing around the corner of your straight stitched side using zig-zag. But you can do either. You can mix up straight and zig-zag stitching at will.

Just ease up to a stop, ensuring the needle is positioned down inside the work when you stop.
Lift the foot.
Adjust to your desired stitch setting.
Lower the foot.
Turn the wheel toward you manually to ensure the stitches will be placed where you want them, and
Slowly engage the power again (via foot or thigh lever).

Hope this helps.

there could be something wrong with the cam that does the decorative stitches, you may want to have the machine checked..

it sounds like something has happened to the cam gears that
do the decorative stitches, which means the sewing machine needs
to be checked..
but before you have the machine checked,, make sure all the
dial that are used for decorative stitches are at the right setting
one missed setting can cause a lot of problems

You need to backstitch over the beginning and end of your stitching, Jodie. When you start out, manually turn the wheel to put the needle into the fabric. Push the foot pedal to stitch forward a little. Now, reverse over those stitches to nearly where you started stitching. Use the foot pedal to go forward again, stitch to where you want to end. Reverse again, just a 1/4 inch or so. Then take the fabric out and cut your stitching.

This backstitching keeps your thread from pulling straight out. You should always use it with straight stitches and general zigzag stitches. Don't use it with decorative or embroidery stitches, though, as it mars the appearance of them. But if you're doing any type of topstitching that uses straight stitches, backstitch in the area that will be in the seam allowance, ever so slightly, or your top stitching will pull out, too.

Hope that helps!