Windows 10 Notepad++ frozen forever. How to get out?

Don't taskkill Notepad++ first. Go to the task manager (open using ctrl+shift+esc) and create a dump file (.DMP) by right-clicking the process and clicking "create dump file". The dump file contains everything Notepad++ had opened in the computer memory (RAM). It will be placed in your temp directory. To open it, open Windows File Explorer and enter "%temp%" in the address bar, then sort by recently modified.

After the DMP file is created, it is safe to taskkill Notepad++.

The unsaved text will likely be somewhere in that file. That file could be several hundreds of megabytes in size, depending on how many tabs were opened in Notepad++.

You can search for known words using a byte editor (hex editor) like HxD and find your text. If the file is small, it might be even openable in Notepad++.

A similar guide for Firefox, but applies here too: How do I recover a form in Firefox *without* installing a plugin?