HELP!! I changed my clients SQL password and now everything is down

My client is working for a media company and they wanted her SQL database, however I wasn’t the one who originally set up the database. So I changed the password to get into the database and as you know, once I did that I got the error code, cannot connect to database.

So after doing some serious research, I realized I had to redo the wp-config.php with the new information, however, that means that WP wants to reinstall.

I have backed up everything on my hard drive. I am so nervous to reinstall. She had a ton of stuff on her blog. So once I reinstall WP, can I add everything back?

Am I going about this the right way?