You are trying with different books, just in case you got a bad one? Or launch Sigil from the Icon (with no book).
Have you run Hardware checks on your system? Sigil (and Calibre) can be a little
demanding of resources at times. Memtest86+ and a drive test are good to do from time to time
The most common culprit is still an 'update' of security software, followed by: 'Helper' utilities