WordPress source code is just like tiny components in an engine. If you tweak it wrong, the engine won't start. Lucky for you that in WordPress the engine won't be broken, so you don't have to worry.
You don't have to start it over. Before doing any changes, you should backup what you will change. So, if your change breaks the whole things, you can always roll back to when it works well.
Then, turn on the debugging option in wp-config, around line 81:
define('WP_DEBUG', true);
If debugging option is turned on, any error or warning will be displayed on your screen, so you will be informed of what you are doing wrong.
If you already know what's wrong, then you should be able to fix it. If not, you can go back here asking, or if you prefer you can hire a professional to better solve your problem.