Please open debug console (Help -> Debug Window -> Console), and run "checkwallet" to see what it returns. It may be related to that if your previous wallet was on a wrong fork. If it reports missing/extra coins, you can run "repairwallet" there and it will resync your wallet with the downloaded blockchain.
I upgraded from 1.5 to 1.8. I'm back to 1.5 again because i was hoping that I'd get my coins back if I returned to the old version. I ran "checkwallet" and got this response: {
"wallet check passed" : true
}. Kinda sucks that I lost all my coins for some unknown reason >.<
Now, you have a few options;
If you received your coins while on 1.5 ; the transaction should be confirmed on both chains. So your coins should still be available. To be sure you can check your address with the block explorer. But; if the person that sent you the coins was also on the wrong chain, and his transaction contained invalid coins, then that transaction was not included in the main chain, because the main chain would reject it as it is invalid in the first place. You should contact your payer, and request he updates his client, and pays you again. Because the coins were not sent in the first place (Even if he's using old client that follows a wrong chain; when he switches to the new version, he will see that the transaction was not accepted and his coins are still in his wallet)
If your received coins are there, but your minted coins are lost : This is normal as your client announced the transactions to the wrong chain. But your coinage is not lost in this case (because your stake was just denied, nothing changes in your accounts). Just wait until it creates coinstake again, and you will receive the same amount.
Welp. I don't have any of my coins anymore. Even at 1.5 I don't have the same address as before. I lost all my transaction history too. I have 0 coins now I only got my coins from mintpal and poloniex. I don't know if they'll give me coins again. Sigh. Such misfortune.
It really sounds like your wallet.dat file got deleted or overwritten. You did back up before you upgraded your wallet right? I sure hope so.... BACKUP ALWAYS!!!
Restore the backed up wallet.dat to C:\Users\"YOURUSERNAME"\AppData\Roaming\MintCoin\
If you don't have the same address it means that the wallet.dat file is not the same one you sent the coins to. IMHO.