You say the accountant "has resigned." Does that mean that the firm or practice where he worked is still in existence, but he is no longer there? If that's the case, the firm should take care of the problem for you.
If the firm no longer exists, you first have to determine whether the loss was reported. You seem to be uncertain about that. It's not clear whether it actually was not reported, or if you for some reason just can't find the Schedule D. It's also not clear whether you had any capital gains, or only the loss.
If there is an amount on line 7 of your 2022 Form 1040 or 1040-SR, and the box next to the amount on that line is not checked, then there was a Schedule D in the tax return. If line 7 of your 2022 Form 1040 or 1040-SR is blank, then the loss was not reported.
Either you, or any tax professional, with your permission, can get a transcript of your 2022 tax return from the IRS. The transcript will show whether Schedule D was filed, and if so it will show the information on the Schedule D.
If you determine that the capital loss was not reported you will have to file an amended return, Form 1040-X, for 2022 to add the capital loss.
Do not file your 2023 tax return until you have resolved the issue, because the capital loss on your 2022 tax return is likely to produce a capital loss carryover to 2023.
