Be sure there are not 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, learn how to access your wallet.
Please be sure that you are carefully and accurately typing your private key. A single wrong character will open a different wallet than is yours.
For reference private key looks like this: