Please be sure that you are carefully and accurately typing in your private key. A single wrong character will open a different wallet than "yours."
An example of a valid private key looks like this:
Do not use the above example private key, and never share your own with others. If you share your private key with anyone, they will have full control over your address, and its funds.
Be sure that you are using the same key / file from when you first unlocked your wallet. Essentially, you need to do the same thing each time you open your wallet.
If you saved another version (e.g., the UTC / Keystore file), it may also be worth testing that as well.
If you think you may have mistyped your private key, there is a tool that might be able to help you further. Scroll to the bottom of https://legacy.mycrypto.com/helpers.html, enter your private key and address, and see if that resolves to anything.