Suddenly my brother xl-2220 is sewing strangely! The top stich is fine, no tensio issues, but when I look at the bottom stitch it isn't right! The stitch is very loose and looks like the pattern a snake leaves in the sand. I've altered the tension but the snaking doesn't change.

Te 'handwheel' is the top knob on the right.
The 'Pattern Selector Dial' is below it.

If you are using that dial and it isn't changing the stich, it must have become disconnected inside.

https://www.manualslib.com/manual/483895/Brother-Xl-3200.html?page=16#manual

https://www.google.com/search?q=brother+xl-3200+only+straight+stitches

..

You may need to get an engineer to adjust the machine internally

If the bottom stitching is a mess it usually means that the TOP thread is not threaded correctly. What happens most of the time is that the thread is not snug in the tension discs. First re-thread your machine with the presser foot UP and when you get to the tension discs (sticking up at the back of the machine) hold your thread like you're flossing and go back and forth in the disc to make sure you have tension on it. You should hear a little click when it's in there. (Test it...floss it on and pull it off so you can hear the click.) If the thread isn't between these discs your bobbin thread will be loopy. Finish threading the machine and you should have no problem now. Also, make sure your upper tension is set at 4 and your stitch LENGTH (on the right front) should be 2.5 or 3 for a normal stitch. Hope this helps. Let me know! Thanks for using FixYa

check that you have threaded it right

Well is your machine skipping stitches? "Not looping" does that mean not creating a stitch? To adjust your bottom stitch you usually adjust the top tension regulator. I can try to help you more in detail check out my forums http://www.sewingmachineryrepair.com/forums/index.php

according to the diagram in the manual, the upper thread tension is set by the wheel/knob on top and just to the right of the thread take up lever and thread guide. If you don't have a manual, go to www.brother-usa.com they have a downloadable manual for your machine.

it is easy to switch the stich modes
there are two wheels on the side on the mchine
one in for manualy stiching *the big one)
and the little one changes the stitch
you have to look at the LCD screen to make sure the numbers correspond

First of all check your needle size and utility, for the tension machine it must be the same on top and bottom example pull the top thread and the lower thread they must be the same.
When sticthing the elastic you must devide the amount of elastic versus the fabric : dise the amount before stiching
Hope this will help
Good day

There is a small screw on the bobbin holder. Tighten it SLIGHTLY. This will increase the bobbin tension.