Thank you for posting in in the Final Fantasy XIV Technical Support Forums.

You mentioned you leave the computer on for several days. The computer should be turned off from time to time. Does this crash occur only when the PC is left idle for days?
Additionally, the problem with the PC not starting could be a symptom of another issue.

You may want to try running a PC cleaning program such as cc cleaner as the issue may not be with Final Fantasy XIV itself. Another option you can try is to delete everything Final Fantasy XIV related from the pc and uninstall it. Once done, please reinstall the game.

Thank you for visiting the Final Fantasy XIV Technical Support Forums