We used to rank #1 for a niche keyword and I noticed that we no longer appear in the SERP. I checked Google webmaster and don't see any warnings /errors. How do I go about finding the reason for this and fixing it?
This is a very open ended question. I would need to be able to see the website in question and gain access to google webmaster tools and analytics.
I can say, if you fell entirely out of Google's results, it is a major error or you were caught doing some aggressive spamming.
If it was a minor drop, it may be normal fluctuations.