There is a lot to unpack here, and not enough context, even location will change a lot in the answer, just as examples:
- Country, some countries (USA...) will have been vastly more impacted by current events than others
- Type of employment (freelance ? Employee ? Intern ?)
- Workplace culture
- Length of the "slacking"
- How short are the deadlines and how important the project is ?
First, remember we are in the middle of a global pandemic, and even if you live in the least affected countries, this is bound to affect your concentration, however you look at it.
For the second point, if you are freelance and deliverable will not be hit, it will be a hard situation, although especially in the worst affected countries, it should be possible to negotiate. If you are a employee, this is a lot more salvageable.
Third, does your boss already know about you struggling to focus ? Or will it be a full discovery ? If he doesn't know, was it because you were hiding it, or because you didn't had the opportunity to bring it up ?
How long are we talking here too ? And was progress totally halted, or just reduced ?
With so many questions left standing, I can only assume, and will just describe two scenario that (to me) seems likely:
- You are in one of the worst hit countries (US), progress severely reduced for 3 weeks, no opportunity to talk to your boss about it, deadline will probably be missed:
You are a human, not a robot, and you cannot be expected to be insensible when everything around you is crumbling.
Plan for a health check to ensure you aren't starting a depression or burnout and get help, a job is nothing compared to your health.
Also, do not describe it as "slacking off", you had trouble focusing, and this is just being human.
- Current events aren't really the matter, you really were slacking off and not just having trouble adjusting to remote working or safety measures:
I have trouble imagining that, and you will probably have to negotiate a pay cut or retro actively take holidays or something. Kudos for being at once so sure about your mental state, holding up in those dire times, and recognising your faults though.
Plan a meeting with your boss now, tell him about the deadlines and how the current events are affecting you, help him understand exactly what the current state of the project is, and establish a solution to get as much on track as possible.
Be honest about what was happening to you, just stay factual if you don't want to delve on it, focus on what you can do to fix this (the project and how you ended up getting late on the deliveries).