On February 3rd, 2021 at block 298,000, the Sia network hardforked to implement the Sia Foundation: a new non-profit entity that builds and supports distributed cloud storage infrastructure, with a specific focus on the Sia storage platform.
It's a great thing! But here you are, and you didn't update in time. Maybe you never even updated from the 2018 fork! You can't rent storage space, host, or send SC to friends or exchanges. For most people who updated ahead of time, it was a simple matter of downloading the latest version and installing it.
Not you though, something went (or is going) wrong. It might be that:
Sia is not syncing
Sia is not forming contracts
Your transactions are not broadcasting
Your transactions are broadcasting, but are not being received at your destination
Sia says you are not up to date
Sia says you are up to date, but you know you're not (sometimes happens with very old versions when checking via the siac update command)
You're on the wrong Sia chain if you're not on 1.5.4 or later. Type ./siac version in your CLI, or use the ⓘ in the top nav bar of Sia-UI to check.
The most effective way to resync is to perform a clean install. The process requires:
your Sia seed
deleting the Sia data folders
deleting and re-downloading Sia
setting Sia up again with your Sia seed
After restoring Sia from your seed and resyncing, it might take your balance up to an hour to re-appear in the wallet as Sia performs more actions in the background.
It's a bit of a long process, mostly because Sia will re-download the consensus blockchain file. But resyncing is something you'll need to do anyway, and a clean install works 100% of the time. Use this guide for a detailed walkthrough of these steps.