Did not file for 2024. Actually thought I filed an extension but did not mail it. Now what?

I recommend that you file your tax return as soon as possible. Hopefully, you won't have a tax liability so that the failure to file penalty will not kick in.

Note that, if you do incur a penalty, you may qualify for a first time penalty abatement. Here is an IRS resource about these penalty abatements: https://www.irs.gov/payments/administrative-penalty-relief

You have until October 15, 2025 to e-file a 2024 return; after that the return has to be filed by mail.    If you are getting a refund, there is no penalty for filing after the April deadline.   If you owe tax due, you are subject to late filing penalties and interest.