I agree with Hollice ... the first thing you need to do is set the tension back to normal and do some straight stitches with a regular presser foot on the machine - just using fabric (no batting) - using regular 50wt cotton thread and the appropriate size needle. If you are unable to get the tension back then your machine may need servicing.
Oh ... did you change the top tension only or did you also attempt to change the bobbin tension?
If you also changed the bobbin tension you will have to get that back to normal as well. Generally speaking, I never change my bobbin tension. Under normal circumstances, it's not necessary.
Once you get the machine back to running a nice smooth and even straight stitch seam with 50wt thread you can start over again.