Your boat's bottom is pretty flat, but does certainly have a KEEL.
The CENTERBOARD, housed inside your boat's KEEL may have jammed up due to mud or other stuff getting wedged in te slot along with the centerboard, or if you were able to get the centerboard down during your initial trip (or a previous owners did?), it is possible that when you went aground, the CB may have been pushed up into the keel slot and the rope pendant used to pull it up may have become jammed between the CB and the sides of the CB housing in the Keel, or it happened as you pulled the boat back onto the trailer. Your CB weighs around 25-30# (just enough to let it drop easily) so a little jiggling from below (either after lifting the boat out of the water by crane or Travel-Lift, or by diving under the boat in the water, I've had to do it on my DS II by diving underneath) may free up the CB and let it drop. Always pull on the CB pendant to raise the CB as the water gets shallow or pull it in as the CB gets pushed up by the water getting shallower, to avoid the line getting stuck again.
Lesson for the masses, not trying to single out anyone, but correcting a minor, yet, far too common error in terms: A "Swing-Keel" is a HEAVILY ballasted (like 300#+) version of a centerboard and usually contains the entire ballast weight of the boat. A winch (often like a trailer winch) is needed to raise and lower the swing-keel. while sailing it actually should be locked In the down position to prevent it from retracting in the event of an unlikely (but still possible!) capsize.
O'DAY never built any "swing-keel" boats.
O'Day trailerable cruisers (192, 20, 222, 23-1, 23-2, 25 ,26, and the 1979-83 version of the 22) all had KEEL/CENTERBOARDS, where a lightly ballasted (as I say, about 20-40#) Centerboard was housed inside a shallow fixed-keel. The Keel contained all of the ballast that gave the boat stability, CB weight contributed very little if any stabilizing force, only helped CB to easily lower out of the keel. The Centerboard on these boats could be easily controlled by a light (1/4"-5/16") line that even a child could probably use to raise/lower the CB, no winch needed.
Now, some may consider the 19' MARINER to have been a swing-keel boat, but (and a former VP of O'DAY once strongly posted this) the Mariner was still a CENTERBOARD boat, perhaps better referred to as a "Ballasted Centerboard Boat". because even though her cast-iron CB weighed 165#, there was 200# of lead in the bilge serving as primary ballast, The CB weight on that boat surely contributed to the stability of the boat when lowered, but she was still correctly a CB boat, not a swing-keel. The newer, STUART MARINE built MARINER now has 250# of lead in the bilge and a mere 75# in the now fiberglass CB. OK, lesson over!
Oh, I get a kick out of the way that O'DAY did such a great job promoting the advantages (and as someone who used to own a boat with a retractable keel, I think they are pretty spot on) of their K/CB designs, yet leaves out the potentially important fact that their K/CB designs need deeper water to get on or off a trailer than the swing-keel boats......... I think the trade-off is mostly worth it!