Will changing theme mess up site?

I had my site hacked and everything is messed up on it. I am using the Twenty Fourteen theme. I inherited the site from someone else who managed it previously. The theme hasn’t been updated in a really long time and I think that’s how the attack happened. The hackers got in through a really old vulnerability (timthumb.php)….that was part of the theme files. Once I got back in, there where 11 plugins that were way out of date and wordpress itself was way out of date. I updated them all. The theme has been compromised and does not work….I have the site running under a default theme which works (but looks bad). The admin login works as well. I did not delete the original theme…..but it needs to be reinstalled / or updated before it will work again (and I don’t know what the previous guy may have modified in it). So will updating the theme mess up the site?