Step 1.Do not panic!
If you panic you are likely to take actions without taking the time to think about what you are doing and could end up making mistakes that make things worse and which you cannot recover from. As long as you have a reasonably recent good backup of your wallet, you should be able to recover full access to your bitcoins.
Step 2.If you have the time, just wait for the sync to complete. The wallet determines how much bitcoins you have based on the transactions it finds during the sync. Since the sync started over from the beginning it can only show the bitcoins that you had back in 2009. If you didn't have any bitcoins in 2009, then it will show a balance of 0. As it synchronizes it will eventually catch up to more recent blocks that have the transactions that gave your wallet your current balance. It will then update the display to show you those bitcoins.
Step 3.If the sync has completed, or if it keeps restarting from the beginning, or if it crashes and never completes, or if it hangs and makes no progress synchronizing for an extended period of time, then you'll need to consider one of the following:
- Figuring out what is wrong with the software on your computer and fixing it
- Loading Bitcoin Core onto a different computer and importing your wallet there
- Extracting the private keys from your wallet.dat and importing them into some other wallet software
Be VERY careful about any advice you get. There are a lot of people on this forum that don't know what they're talking about at all. They'll either just repeat something they think they heard previously or they'll just make up some advice. If you aren't absolutely certain that you understand the implications of what you are being told to do, then you should either wait to get confirmation from a trusted source, learn more about what is being suggested, or find someone that you trust that has the knowledge and experience to assist you. Do NOT give your wallet.dat or private keys to ANYBODY.
The
MOST IMPORTANT thing you can possibly do at the moment is make absolutely certain that you have good safe backups of wallet.dat. If you have multiple copies, from different time periods, then make sure you know which one is from when and that you don't get them mixed up or accidentally overwrite ANY of them.