First up: verify that your balance isn't actually in your account:
You only need your address in order to see your balance. It is not recommended that you enter your private key anywhere if you just want to check on the balance or see incoming / outgoing transactions.
https://etherscan.io/. Paste your address into the search bar and it will pull up your address and transaction history.
https://ethplorer.io/ allows you to easily see token balances and transfers.
You can also see some tokens on https://etherscan.io/, especially with their new "Token Transfers" tab.
https://gastracker.io/. Paste your address into the search bar and it will pull up your address and transaction history.
You can also enter your address only on the View Wallet Info page by selecting the "View With Address Only" option at the bottom. Note: This does not mean that you can transfer those. Remember, you must have the private key in order to access your account.
Secondly: Make sure you are on the correct chain on MEW (ETH vs ETC vs Testnet)
For example, in order to see your ETC and interact with the ETC chain, you must first be on an ETC node. Select "ETC (Epool.io)" in the node switcher in the upper right corner of https://www.myetherwallet.com/.
Then you can unlock any wallet send a transaction in ETC, or see your ETC balance by clicking the "View Wallet Info" page -> "View with Address Only"
In order to switch back to ETH, simply change the node back to an ETH node in the upper right.
Third: Make sure you actually were sent your funds
When you search your address on the blockchain explorer, you should see transactions. If there are no transactions in (green), that means that you never received them. That menas etiher they weren't sent or they were sent to a different address.
If you see transactions in but you don't have a balance, it means you must have sent the ETH out or your account as been compromised. Did you send them to an exchange and forget about it? Did you get phsihed? That's something you will have to determine.
Check the TX hash, is it to the address you are checking?
Check in with the person who sent the ETH to you. Are they sure that they sent them? Where to? Do they have a TX Hash?