Forgot my key from another computer, computer was stolen

I believe, you have lost your MFA device(yubikey etc).

Here is the AWS documentation, which provides step by step process for regaining access to your aws account.

If this doesn't help, then I'd suggest you to reach out to AWS support from here, they will surely help you out.

Refer My MFA device is lost or stopped working or I need to contact AWS Support for sign-in issues for additional help and support.

If this is for SSH key, refer this re:Post Knowledge Center Article, which has outlined steps to get out of the situation.

Hope this helps.

Comment here if you have additional questions, happy to assist.

Abhishek