I'm assuming that you've deleted them from your computer's Recycle Bin. (Indeed, I'm also assuming that you're referring to a Windows-based computer, rather than to, say, an Android tablet or an iPad).
You need a file recovery program, such as this popular freebie:
https://www.piriform.com/recuva
However there's a risk that downloading and installing it will wipe the very files that you're trying to recover. (That's assuming that the missing files are on your hard drive; that obviously won't apply if they're on, say, a media card).
So, if the files are really important, the best way forward is to install Recuva onto a different machine and then network the two machines together in order to search for the missing files. With the massive size of modern hard drives though, it's probably fairly unlikely that installing Recuva would delete your missing files, so you could take a chance and simply install it onto the same computer anyway.