remember when ever you change the tension dial you have to raise and lower the presser foot to ensure the new tension you have set is engaged.
re threading the machien completely top and bottom fixes 99% of issues.
If that does not work:
Have you changed the needle since it was working okay?
Check the needle you are using isn't bent.
if you changed the bobbin, is the thread on that wound correctly, ie firmly and evenly.
is the bobbin in the race with the thread coming off in the right direction. that makes a difference for some machines.
take it apart as much as you are able and give it a good clean out. I have had manay a student save themselves a repair bill by giving their machine a bit of TLC and a good defluff.
If all that fails, then take it to a good repair person for a service.
Hope you get it sorted. I was two days from my final City and Guilds submission deadline when My machine started sewing staircases instead of a satin stitch because some thick stuff knocked out the timing so the needle was swinging side to side inthe fabric instead of above it.
It had to go to the sewing machine hospital.
Lucky for me I have a second sewing machine (that's I usually only ise for embroidery) once I had snagged the foot box and found the manual I was able to keep sewing and meet my deadline. the look on DH face when I reported the issue withthe main machine was actually quite endearing. He looked genuinely distressed for me. I triumphantly told him I had switched machines... see honey that's why I need two sewing machines... Okay so I actually have three... and a protable so that's four isn't it... :)
Hope you get it sorted, I wasn't happy until I got my main machine back. Using a different sewing machine is like driving a different car. you don't know where the bite point is on the clutch ( foot pedal) I looked constantly worriedwhen using the other machine!
Becks