when I am writing

[mtt349]

and suddenly comes power outage, will I lose data?

[Mojoshog]

Yes if your post hasn't posted yet and your computer shuts down I think you will loose it.

You can use the Save as Draft feature to overcome this but otherwise you'll have to start over.

[hicham1976]

it was a tornado and storm today 

[CharmedPhoenix]

If you don't have a uninterrupted power source or a laptop you probably will.  However some computers are good at trying to save things when ungracefully shut down so look for recovered files.