When a Clan Leader hasn't logged into Clash of Clans for 60 days, the process of selecting a new Leader begins. After a total of 90 days of inactivity, the Leader role is given to an active Co-leader or Elder of the Clan.
Support cannot rotate leadership in a Clan.
The Clan Leader has 30 days to come back to the game before his status is given away. To stop the demotion process the Clan Leader only needs to login to the game.
The status of Leader is given to the next longest standing and highest ranking active member in the Clan. The new Leader must be at least an Elder and has to have played Clash of Clans recently.
Clan members and the Leader will see messages about the process in Clan Mail in their Inbox.
Below are the reasons why a demotion process might not start:
Leader hasn't been inactive for 60 days
It has not been 90 days since the current Leader became a Leader
There are no Elders or Co-Leaders who can be promoted to Leader
If rotation can't happen in your Clan, your best option might be to find or create a new Clan. Consider using the Clan chat to connect with your clan mates and send them in-game friend requests. This way, you can invite them to a new Clan and start afresh.
Consider connecting with the Clan Leader on social media if you haven't done so already. If you can contact them through some other communication channel, you can let them know that the Clan needs help.
A Clan Leader can willingly give his Leader status by logging in and promoting another Clan member to Leader. Leaders can't leave Clans unless a new Leader is already in place.