1. Do you know your BTC Address, Go to a btc block explorer and verify the funds match the block explorer
2. Open the Wallet.dat with the Bitcoin Client Software, look under Receive coins Tab and verify that is the correct wallet.
Then Dump your Private keys and import into another Wallet.
steps: Open the wallet, Click on Help, click Debug Window, click the Console tab,
For unencrypted wallets: beside > type dumpprivkey (BTC Receive Address) , hit Enter key.
For encrypted wallets: beside > type: walletpassphrase "your walletpassphrase here" 600 , hit enter, then type dumpprivkey (BTC Receive Address) , hit Enter key.
Copy and Paste the private key along with the BTC receive address into a text document.
Repeat those steps for each of your Receive address that had BTC in them, once you have finished.
Go to your new BTC Wallet installation on another pc that you have newly synced.
steps: Open the text document created above.
Open the wallet, Click on Help, click Debug Window, click the Console tab,
beside > type importprivkey (Paste your Private Key here) , hit Enter key.
Then Close & Restart your wallet with bitcoin-qt.exe -rescan
The Amount for that address should be available after the rescan finishes.
Repeat until you have imported all of your private keys into the new wallet.
You should be good to go.
If the receive address are not in the wallet.dat when you view it, you have the wrong wallet.dat
Or the wallet.dat is so corrupted , that the address is not viewable.
If that is the case look for a older wallet.dat that has the address viewable, does not matter if it was before funds were transfered , what you are looking for is the private key for that address, and a rescan after import will have the updated amount of BTC in it.
If you don't have a wallet.dat files with the receive address so that you can export out the private key, there is not much you can do.
FYI:
If you are into python programming:
Then check out on how to extract the keys.
https://bitcointalksearch.org/topic/pywallet-help-need-to-recover-private-key-from-walletdat-615391https://github.com/jackjack-jj/pywalletFYI2:
If this was a windows install, you can use shadow explorer to see if their is a good shadow copy of wallet.dat to restore,
https://www.shadowexplorer.com/