My son would rudely say: RTFM - read the f*** manual - but it's true, your manual should tell you. I don't know if you can adjust the tension by turning the dial by hand - some of my machines have to be done that way - or in the control panel - like my Designer. But whatever way you have to use do it in small steps.
If the problem did arise suddenly, are you sure the machine is threaded in the right manner? More often than not problems are due to incorrect threading, or a bobbin inserted upside down.
I hope you can find a solution to your problem quickly.

you have pm