First off, ALWAYS leave a paper trail. An incident report definitely should have been filed. A head injury has the potential to be dangerous and even life-threatening if you were to get a subdural hematoma, and the symptoms may be delayed if you have a slow bleed. Getting the name of the front desk attendant would also have been good.
As far as not being able to get a copy of the incident report, that's what cell phone cameras are for
I would have, as suggested, also gotten pictures of the headboard and any bump or bruise on your head.
I realize that this is after the fact, but I would still document the incident and send it to Hilton corporate. JMHO