I have had a small home sales business, carrying inventory, but the parent company went bankrupt in 2018. How do I reflect the loss of value of that inventory, and where?

[barbaranoel]

For the prior 2 years I filed losses since no sales but cost to maintain accounts, advertise, etc.  Inventory is about $3,000 value before their bankruptcy - worth about 25% of that on the current market. 

[Uther]

Unfortunately, most taxpayers are on the cash basis, so there is no way to simply "down-value" inventory. Even though you may be quite certain that it's lost some value, you'll need to sell it to establish just how much of a loss you actually have. 
If you sell for less than what you paid for it, then and only then will you have a loss for tax purposes.