Make sure there aren't any spaces at the beginning or end of your private key. We validate based on the number of characters. A private key has 64 characters – count to make sure you have all 64. If you saved another version (i.e. the UTC / keystore file) it may also be worth trying that file as well. For more information about how to use a keystore file, see How to Access Your Wallet with MEW Portfolio.
Please be sure that you are carefully and accurately entering your private key. A single wrong character will open a different wallet than is yours.
For reference, an Ethereum private key format looks like this:
afdfd9c3d2095ef696594f6cedcae59e72dcd697e2a7521b1578140422a4f890
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