Did you backup your files to an external hard drive? If yes, just copy them from there to your computer.
If not, you need to download them again.
Do the cards show in your collection? If yes, great, all is good.
If not, maybe you need to synchronize with server or sign in as a different user.
Go to settings and make sure advanced settings is checked.
Click synchronize with server.
If that does not fix the problem, then go back to advanced settings (this stays selected, until deselected manually).
Click login with another account. Put in your username and password.
Warning: do not put your password here