Put in a claim and see what they say.
Though before you do so, I’d recommend that you contact a Loss Adjuster in advance, as those guys are good at negotiating claims for customers (for a good cut of whatever you might eventually get!). They’ll tell you straight off the bat whether you’re likely to succeed, as they’re used to dealing with insurance companies and customers’ claims.
I don’t think your policy would cover the damage, as the cause arose before the date the policy commenced, but I could be wrong.