If the IP of the site is already known (like OP), or is discovered, CloudFlare on its own is of no use at all to block a (D)DoS attack. Absolutely zero.
For pre-emptive measures or to put up right before you switch your site to a new IP: it can work, absolutely. (Indeed with limits on the free plan btw.)
For sites already under attack, it's usually useless (exception: the attacker has no clue what he/she is doing, or the sw is crap.)
to switch to CF unless you also move it to another server/IP.
CloudFlare is quite easy to bypass with the default configuration that most people use, always pay attention to that as well.