Files gone

As the title says, there are missing files on my hard drive. This is my first post here, so please let me know if I made any mistakes. 2 weeks ago, I decided to upgrade my PC with a new video card and a new processor. As I am 15 years old and do not know much about how to handle those things, I...

As i said i uncovered my old HDD to take a look what's in it and get some storage. And i tried to open some folders and it said ''acces denied'' (i have different lang. on my windows, i hope i have it right) What can i do with it?