Google ranks pages based on relevancy and relative importance.
* Ensure that you are still indexed in Google.
That verified, your ranking drop could be due to penalty or simply due to competitors gaining superior backlinks than yours.
* Have you been gaining links in any unnatural way against Google TOS? ( paid links, sponsored links, link schemes, etc etc ). If yes, clean the mess and file a reinclusion request throí Google webmaster central console.
* Compare your current backlinks with historical backlink data when you used to rank at top. You could have lost few high quality links. Replenish those.
* Check your backlink anchor text profile. Does it appear natural? Can your backlink profile pass a human review? If not, bring some variety in your backlink anchor text profile.
* Sometimes, simply not updating your site for too long can lead to competitors overtaking you.
* The most dangerous competitor is the one who has not arrived on the scene yet! So IMHO, itís time to pull up socks and build some quality content + links to regain lost positions.