Your Sia seed grants full access to your wallet and balance of Siacoins. For various security reasons, the Sia team does not and is not able to have access to any user's seeds. We cannot recover your seed for you.
Losing your seed means permanently losing access to your coins.
But you might not be totally stuck.
A way to view your seed
This is a way in which you can locate your current seed using Sia. If your wallet happens to already be unlocked, you can recover the seed using a command. In Sia-UI, click the > at the top to access the Terminal.
Follow these steps if your wallet is currently unlocked.
Click on the Terminal tab
Type: wallet seeds
and your wallet's seed will be presented to you. Make sure you safely store it right away.
Additionally, if you've updated to 1.4.0 or later, you can access your Sia wallet, click on More, and choose View Seed.