Backing up my Wallet is no longer an option because I cant open it.
Never do anything without a backup.
Please read the quote again:
Mate, make sure you make a backup of your wallet.dat before taking any actions.
Location will be: C:\Users\YourUserName\Appdata\Roaming\Bitcoin\wallet.dat
Your wallet (storage of your keys) can still be backed up, since it basically is a file on your computer.
Just copy the wallet.dat file in the above path to a save location (preferably not only on your pc).
I understood that I have to do -reindex.
But I do not know how.
I guess the easiest option is option #2 mentioned by mocacinno.
Based on your question, i assume you are using windows.
Right click on the shortcut -> properties -> add
" -reindex" (with a leading space, without the quotes) in the "target" field at the end.
You want it to look like:
"C:\Path\to\bitcoin\core" -reindex
Obviously the path itself will be different on your system.