I used my trunk all morning and then for some reason now it won't lock. I can close it all the way but it will just come back open, it won't stay closed/locked. Any ideas? I used a pen to replicate the trunk bar and the lock spins around the pin but won't stay shut

Try to lube it up. Also check to see if its not aligned anymore, something maybe loose.

Well it all lines up. It closes like normal it just doesent lock. The mechanism closes just wont stay closed. Should I try and replace the lock mechanism on the trunk lid?

yea i guess... you could try a junk yard first, not sure how much they cost at the dealership

I assume you checked the obvious which would be the manual trunk release accessed thru the rear seat pass thru? That release can get stuck in the open position.

There's also a release inside on the trunk itself on the side opposite the Acura logo. Check that too.

Look at the latching mechanism in the trunk lid, There is a hook like peice of metal that should have its opening pointing down. If it is not, place the key back into the trunk lock and using a screwdriver gently pull the opening of the hook downward

Key into trunk lock? Am I missing something here?