Lightroom CC v10 Downgrade

At the very least try this: 

Try resetting your Lightroom preferences:
https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html
Also see: https://community.adobe.com/t5/lightroom-classic-discussions/important-steps-for-manually-resetting-preferences-omitted-from-help-article/td-p/12972518

Quit Lightroom Classic.
macOS: Press and hold the Shift + Option keys.
Windows: Press and hold the Shift + Alt keys.
While holding down the keys, launch Lightroom Classic.
When asked, select "Yes".

Agreeing with @Ian Lyons , it would be better to describe the problems you are having now, and then fix those problems (we can help), than downgrading. A problem with downgrading is that your LrC v12 and LrC v11 catalogs cannot be used in LrC v10.

You haven't mentioned what you mean by 'nothing works'. So, it's a tad difficult for us to provide advice that could potentially get you up and running again.

As for getting your old versions back, I suspect you'll need to contact Adobe directly rather than through this community focused forum. Therefore, you might be better served asking how to contact the correct people Adobe at https://community.adobe.com/t5/account-payment-plan/ct-p/ct-account-payment-and-plan?page=1&sort=latest_replies&lang=all&tabid=all