Hello, if you didn't fully delete your FB account (temporarily deactivated), then you can restore your FB account.
If you have already permanently deleted your FB account, you will not be able to un-delete it, and subsequently reconnect to your Instagram account.
If you followed these following steps before deleting your FB account, you would be able to access your Instagram account.
Before deleting your FB account, you would have needed to have logged into your Instagram account separately (not with FB) and gone into account settings > add a working email ID to the account.
Now when you delete your FB account, it will automatically disconnect from the Instagram account, and you will be able to login to your Instagram account with the new email ID connected to your Instagram account.
If you are unable to recover your deleted FB account, then you will have lost access to your Instagram account if connecting via FB was the only login option you had.