If you managed to import an address and its private without triggering the built-in rescan function to update the current balance of your wallet, then you will need to close the wallet software completely, and restart it with the rescan option from the command line.
The "wallet.dat" data file maintains a current balance for your addresses. Every time that a new block is downloaded, any payment to you is noted and the balance in addresses you own is updated in this local file.
However, if your local client is in sync, having already downloaded the entire blockchain, and you then import a private key through a third-party utility, the Bitcoin client doesn't go back in history searching for old payments to this new address. You must close Bitcoin, and relaunch it with the command-line option "bitcoin-qt -rescan". This will start up Bitcoin in a mode where for several minutes it will re-examine the blockchain and attempt to detect any missing payments to you to correct your balance.
After doing so, you should be able to go to the "send" tab and select "inputs", and see the individual payments previously received by this address which contribute to your balance. If you still don't, you should look up that specific address in a block explorer site to see if it really has coins - your old wallet may have your bitcoins in other addresses than the one you think, due to "change".
Here's another old post of how to do this:
You need to add a command line option at the command line. On windows, open "command prompt" under accessories.
Here is a typical command line session for 32 bit windows (blue is what you type):
C:\Users\user1>cd \
C:\>cd "Program Files"
C:\Program Files>cd Bitcoin
C:\Program Files\Bitcoin>bitcoin-qt -rescan
Then Bitcoin will start with the wallet splash screen displaying "rescanning..." for a minute or two.
