I haven't played ranked for a while and I just noticed that my rating has decreased is it beacuse I haven't play ranked for a while? or is it another reason?

Yes, you lose ELO if you don't play for like a few weeks.

Yes, you will lose rating if you don't play for a few weeks, but it cant get below 1400 (or maybe that was only for the end of season 1)

Does it continue to decrease if i still don't play ranked?

I believe it decreases by around 15~20 every 2 weeks of inactivity in ranked queues

Yeah, I haven't played on NA server for few months and my elo fell on 1400.

Yes it will decrease if you don't play ranked games for a few weeks. If you play normals inbetween doesn't matter; your elo will still drop if you don't play atleast one ranked.

3 weeks, -25 rating, till 1400 then it'll stop
old ranked 5s and 3o3s will be 1400 for all :P

-1 rating per hour you are not actively playing.. RIOT wants to force you to become an addict!