Theft losses of personal-use property are only deductible if you itemize deductions, and only the amount exceeding 10% of AGI can be deducted. The amount also needs to exclude whatever insurance payments you got, and exclude $100 per theft event.

As a concrete example: say your $1500 laptop gets stolen from your apartment. Your renter's insurance has a $1000 deductible, but covers the rest, so they pay you $500. Your loss for tax purposes is the $1500 - $500 - $100 = $900. You subtract 10% of your AGI from this, and deduct what's left. Which is most likely "nothing", in this case: if you're in a situation where it's making sense to itemize deductions at all, the chance that your AGI is below $9k is fairly low.

But also:

1. Starting in 2018, theft losses of personal-use (as opposed to business-use) property are only deductible "to the extent they’re attributable to a federally declared disaster" (see IRS Publication 584). So in the laptop example it would need to be stolen as part of the general chaos of some sort of federally declared disaster or something. But this does not apply to thefts of "income-producing property" e.g. many stocks or other investments, which brings us to....

2. There are some special rules for Ponzi schemes that I don't know in detail. https://www.irs.gov/publications/p547#en_US_2020_publink1000... has links you can chase if you really want.