I was adjusting the Sizes in the ControlPanel Theme Tweeks area and at one point my SideBar became completely unresponsive. I could not create a new tiddler. I could not open the Control Panel again.
After scrutinizing my file system for permissions issues and finding none (it is a node tiddlywiki), in a desperate attempt to recover my wiki I imported the $:/ControlPanel into the non-functioning wiki from one that was working normally. After resetting the sizes (Story right, Story width, Tiddler width, Sidebar breakpoint, and sidebar width) to match my functioning wiki, I was able to recover the sidebar function again.
I don’t know how or why this happened, but I thought I would throw this out there so you gurus could speculate and educate the neophytes like me.
For future reference when one get in a pickle like this there are a few strategies that may help recover
Import or drag and drop the tiddler you broke (as you suggested)
Go to tiddlywiki.com and lookup the shortcut keys to do things you can no longer do through the user interface
Install a utility package that may help you fix the problem
One example I use is edit-recent-sidebars.json (3.2 KB) which provides tabs in the sidebar for recent, system and temp/state tiddlers but also has an edit button, so you can open a tiddler directly in edit, rather than view it first. This complements the next tip.
You may want to start your wiki with a working tiddler in the address bar https://tiddlywiki.com/#Home so a problem tiddler is not opened in the story.