@Kim3128 If it's impossible for a guest to get to your place, I would offer a full refund. It's not like the weather is just inclement so they won't be able to do outdoor activities they planned on.
And while bad weather is not covered as an Airbnb extenuating circumstance, they are likely to refund the guest anyway if roads are closed and the guest pursues a refund.
So IMO, it's better for the host to offer that refund unsolicited- it makes you look like a good host to the guest, so they may book with you again, or recommend you to family or friends.
You can also first offer to let them switch their booking to other dates. I'd say to also offer to refund any cancelled dates you can rebook, but if your place can't be accessed due to impassable roads, you aren't likely to be able to rebook, anyway.