Assuming you did not pull the power cord or run out of battery, this probably indicates a drivers problem on your computer. To solve the problem I recommend reading Audacity Manual .
You can usually solve this as per Error: not well-formed - #2 by Gale_Andrews
If you prefer, please attach the autosave file. Please see here for how to attach files: https://forum.audacityteam.org/t/how-to-attach-files-to-forum-posts/24026/1
Gale