Remember that there are generally two accounts involved in staking - the stash and the controller account. (Learn more here.)

In order to stop nominating, you need to have your controller account active in Polkadot-JS to perform controller-based actions such as stopping your nominations. 

If you're unsure which one is the controller account for your stash, you can see all of your stash/controller combinations on the Account Actions page. Your controller account will be listed to the right of the stash account.

If it is not active, it will display only the raw address of the controller, as shown below. (This example is shown on the Westend testnet but the result will be the same on Polkadot).

To make your account active again, ensure that the correct network is selected for both your stash and controller accounts in the browser extension:

Then after refreshing the page, it should display the account nickname:

Once you have found the relevant controller, make sure it is also active on your accounts page. If you can't see it there, you can restore it from your mnemonic seed or JSON file or attach your Ledger, if that is where you keep this account.