Using the wrong chain after a fork
How a fork works
When a blockchain hardforks, a new version of the blockchain is created while the old is left behind. This typically happens at a certain block height.
When Sia forked, it occurred at block 179,000 and means that we now have a new chain that the Sia team will be supporting. There's also the non-fork chain that can either be maintained by a separate group, or forked again to create a new project. Here's a visual representation of the recent Sia fork.
What this means for users
Frequently, after a fork, users of a blockchain don't update right away. This means that while the main network is operating on
version Y, some users are operating on
version X. If you're on
version X and try to send coins to an address that's using
version Y, they won't go through.
The same is true for Sia. If you have not updated to 1.3.7, but send coins to someone who has updated, the transaction won't be received on the other end. It will look like those coins have disappeared.
Since all exchanges that trade Siacoin have updated to 1.3.7, sending coins to an exchange from a pre-1.3.7 wallet will result in the loss of those coins.
Your coins are safe
You have not lost your coins. At least, not your main network Siacoins. Since you were on the old chain when you sent them to a 1.3.7 wallet, you sent and lost your old chain coins – the coins that would have been spendable on forks of Sia.
Once you download and update to 1.3.7, your coins will reappear. Any coins held in your official Sia wallet as of block 178,999, just before the hardfork, will be accessible in your wallet once you update.
If you're having additional issues that require a clean install of Sia, use these instructions.