Thread breakage or shredding in a Brother PE770 Embroidery Machine can be caused by several factors, including:

1. Using the wrong type or weight of thread for the project
2. Poor quality or damaged thread
3. An improperly threaded machine
4. A damaged or dull needle
5. A damaged or worn-out bobbin case
6. Tension issues
7. The machine needs to be cleaned and oiled.

Here are some steps to try to fix the issue:

1. Check the thread spool and make sure the thread is not tangled or twisted. Try using a higher-quality thread.
2. Re-thread the machine, making sure the thread is properly seated in the tension disks and threading path.
3. Check the needle to make sure it is not damaged or dull. Replace the needle if necessary.
4. Check the bobbin case for any signs of wear or damage. Replace the bobbin case if necessary.
5. Adjust the tension as needed. Refer to the machine manual for instructions on how to adjust the tension.
6. Clean and oil the machine as recommended in the manual.

If these steps do not resolve the issue, it may be necessary to take the machine to a professional for servicing.