If the loss occurred in 2019, it has to be claimed on your 2019 return.
A loss is only claimed when it is "realized" or made real. This may be tricky to figure out. For example, if the stock is worth 1 cent per share, that's not "realized" until someone buys it from you at 1 cent per share. Often, a broker will do that for you so you can realize your loss.
However, it should show as a sale on a form 1099-B. Fidelity handles a lot of 401(k) and IRA accounts -- did this loss occur in a qualified retirement account? If so, you can never declare the loss. The whole point of a qualified retirement account is that whatever happens inside the account in between making a contribution and making a withdrawal has no tax consequences. Your "loss" is the fact that you have less money to withdraw when you retire and so you pay less tax on those withdrawals.
If this was a regular account, you may need to call your broker to determine whether the loss has been realized yet and why it wasn't reported.
