If the top thread is lying on top of the fabric, the tension on the needle thread is way too tight - it's like a tightrope and all the bobbin stitches are just draped over it.
It sounds like you have the tension turned up as high as it will go? Dial the tension back a little about a quarter or half-turn at a time and see if you don't start getting a balanced stitch again after a few turns.
If not, then it's possible that the thread started hanging somewhere - it got wrapped twice around the tensioner or something.
Try re-threading the machine, making sure that the thread goes in between those tension disks, not outside them and that it flows in a clear path from the spool all the way to the needle - not wrapped around a spindle or anything.
If that fixes the problem, great. If not, then maybe the tensioner is improperly assembled or there is something gunky in the tension disks that's gripping the thread too hard.
The manual can show you how to take the tensioner apart - clean all the parts, especially those two silvery disks, and put it back together.
If you don't have a manual, PM me and I will send you a pdf of the owners manual and another pdf of the tension assembly. It's very easy to take apart and re-assemble. :)
Have fun!
Elizabeth