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.
For example, Sia is forking around February 3rd, 2021. When Sia forks, it will occur at block 298,000 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 be maintained by a separate group if they want. Here's a visual representation of the upcoming 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 still 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 1.5.4 or later, 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 will update to 1.5.4 or later, sending coins to an exchange from a pre-1.5.4 wallet will result in the apparent 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 1.5.4 wallet, you sent and lost your old chain coins – the coins that would have been spendable on forks of Sia.
Once you update to 1.5.4 or later, your coins will reappear. Any coins held in your official Sia wallet as of block 297,999, just before the hardfork, will be accessible in your wallet once you update Sia and sync the new version of the blockchain.
​Download the latest version of Sia now.​
Now that you're updated, to sync the new version of the Sia blockchain you first need to delete the old one. Quit Sia.
If you're on 1.3.7 or earlier, you can easily find these folders by clicking Show Sia Data in the About tab.
If you're on 1.4.0 or later, use the Info button at the top of Sia.
Then click Open Data Folder.
Once you find the /sia folder:
delete the consensus and transactionpool folders. Reopen Sia to let it sync the correct version of the blockchain.
Special considerations for miners
Some forks come with no changes to the mining algorithm for their network. In this case, like with the 2021 Sia network hardfork, you just need to make sure your mining pool upgrades in time (they did).
Miners might be affected in other ways by a fork. If you've set up your miner to mine a certain coin, a fork might change the algorithm associated with mining that coin. An example of this was the 2018 Sia network hardfork, which invalidated all ASIC Sia miners except those produced by Obelisk. If you were using a Bitmain A3, Innosilicon S11, or any other non-Obelisk ASIC miner, you were no longer mining the primary dev-supported Sia chain after the fork.
What this meant for you was that your miner was still working, but it was mining the pre-fork version of Siacoin, usable only on pre-fork versions of the Sia network. These coins that are mined were not visible in your wallet until you upgraded your node.

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Navigating the 2021 Sia hardfork

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Outline

How a fork works

What this means for users

Your coins are safe

Special considerations for miners