If you can’t drag and drop widgets in the admin pages after you have changed your code, look at the enqueued javascripts and plugins you may have activated in the meantime.

1. Search for wp_enqueue_script calls. Probably you have added jquery-ui to your code. Be careful with it. Check out the folder at /wp-includes/jquery/ui. Use already registered scripts like:

2. Deactivate plugins temporarily you have added or upgraded recently to see if this resolves the problem. If this works, re-activate them one-by-one to find the bad code.