Can you recover a letter under notepad if the computer reboots?

No. Some programs, such as MS-Word, save information automatically, every few minutes; if you use google Docs, information is saved online every few seconds. But Notepad doesn't have such a feature. Some of the Notepad substitutes may have such an option. Note that in general, Notepad is a very limited program, so you may want to get a Notepad substitute for other reasons as well, not just for autosaving. For example, Notepad++, while NOT having autosave (at least in the version I am currently using), allows you to open multiple Windows, search among multiple Windows, allows multiple undos, and in general has a neater editing interface - for example, Ctrl-Backspace works, to delete a Word to the left, whereas in Notepad, this doesn't work.