Don't panic! Most likely your NXT is still there, it just hasn't shown up in your software. You can still see your account balance through a block explorer like
https://nxtblocks.info/#section/blockexplorer_accounts or
http://mynxt.info/blockexplorer/index.php.
If the block explorers show your account balance correctly, check if it's due to these two reasons:
1) In Nxt,
your passphrase is your account. If you type a different passphrase, you'll end up in a different account. An unused account has 0 NXT, which could be what you're seeing. Check the account's address; if it's wrong, that means you mistyped your password. Log out and type in your passphrase again carefully.
2) It could also be that the blockchain hasn't finished downloading yet, wait until it's done. The blockchain is a record of all transactions made since the beginning of the Nxt network, and your account balance is calculated by totaling all the transactions into and out of your account. If it hasn't downloaded to the point of the first transaction into your account, that total will be 0. If it hasn't finished downloading completely, the total may not be up to date.
If the account number is correct and the blockchain has finished downloading, then it's due to something else. Make a new thread about it in this subforum (but never reveal your passphrase; again
your passphrase is your account), the community will try to help you.