Depending upon the choices you made if you went backwards from W10 to W8.1, specifically, if you chose to format your hard drive, most of the programs and data you had in your W10 may now be gone.
And if you've chosen the same option and used W8.1 factory Recovery Media to reinstall your W8.1; same thing. Your data is now most likely gone.
You can take your computer to a local Computer Pro and attempt data recovery, but it's unlikely to work now that you've formatted your hard drive twice! It still may be worth a try, but I would consider sending it out to Professional Data Recovery and spend the money now, as fooling around with various Data Recovery softwares to fix your mistake of not backing up your data in W10 is going to turn out very bad for you in most cases.
Cost is typically going to be $550 and up. If you decide to do this, and if the stuff you lost is worth it financially,
take a look here at this link to contact the best 2 companies to do this:
Forced Windows 10 update damaged my attached external USB hard drive This is one sacrifice you would need to make; the financial one. The other one, is that you would need to pull that Hard Drive with W8.1 out of your current computer, and
STOP USING IT IMMEDIATELY!! The longer you continue using it, the less chance you have of recovering anything from it! You need to purchase another 2nd Hard Drive and install your W8.1 or W10 whatever you want on that, reinstall your programs from source media or download sites, and begin using that new drive for your computer while you decide if you want to spend the money to recover anything from that original hard drive that you had the OS change on. If you do, you should know if you send it our for Professional Data Recovery as I suggest, you are looking at from 4-8 weeks or longer to get it back. Yet another reason to buy a new hard drive since you cannot and should not be using it in your computer now any further!
I think we have brow-beaten you enough about the importance of data backup now!
Let us know how you get on or if you have any more questions.
<<<BIGBEARJEDI>>>