Sorry for this long post... I am in need of URGENT help. I made a mistake, and in the process of fixing it made other mistakes. I am frightened that I might not be able to get things working again. Here is what happened to the best of my knowledge:
I upgrade my wordpress version using the "automatic wordpress upgrade" feature where I follow on screen instruction on upgrading it, so I never end up downloading the upgrades to my hard drive. Just a couple hours ago I wanted to make a change to my functions.php file in wp-includes/, and, forgetting that I had old wordpress files on my hard drive, I overwrote the new file on my web server. This brought about a PHP error on every page.
Now I don't remember the exact details here, but as far as I know I downloaded the final version of wordpress and replaced the wp-includes folder on the server with the newest ones. I go on, everything seems to work, but now all my plugins in the plugins settings show errors, and disappear.
To make things short, I kept trying to do things by uploading different files to the webserver, coming up with different problems every time. I eventually deleted the entire wordpress directory and uploaded the original files (old wordpress version) from my computer. Now, only the homepage works, and other pages cannot be found (I guess different settings due to database differences). I go to site.com/wp-admin/, log in, and it says database needs upgrading. I did. And now... it doesn't let me log in. The error:
Firefox has detected that the server is redirecting the request for this address in a way that will never complete."
PLEASE HELP ME! I have no idea where to go from there. I know I am foolish for not keeping an up-to-date backup. I could give someone access to my web server if needed so someone can fix things up to the way they were, with the latest version and all my plugins working. I can pay someone to do it if need be. This has been (sorry to say) one of the shittiest webmaster days of my life.