Sounds like a tension issue.
Check your tension units and thread guides are clean and free of lint....a rag moistened with methylated spirit (denatured, surgical etc) run between the disks will help.
Normal factory tension setting = 3 for all dials although resetting this style of tension unit is, frankly, awkward, so if it ends up being other than 3, don't worry too much, just write it down somewhere handy.
Start with all at tension 3 and try using 4 different colours of thread and sew a scrap to see the balance of the tensions to more easily locate any that are loose.
Ideally, the 2 needle threads should appear as secure dots not at all "loopy", while the looper threads should meet neatly at the edge and be a firm join without pulling at the edge on one side or the other.
Get the needle threads right first, then small adjustments to balance the looper threads....after a few adjustments you will probably have it back to normal.
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