I'm probably not as helpful as you may think.
I encountered this glitch too on pc and it happened because I accidentally re-entered the sewers. Although I was still able to enter the mansion after looking up online its location--only the objective marker was gone. After I entered the house the next portion of the mission started and fixed the objective problem. I'm not sure how to help if it won't let you inside.
This thread is about the glitch that leaves the player with a flashing red health bar, a limited amount of blood and a lack of lock-on and vampiric senses.
Long story short, the bug happens when a player ragequits.
When the player dies, I assume the game saves the playerstate as dead and will make them come back to life on the next respawn which would normally be after an upcoming loading screen. By returning to main menu and joining back, the game doesn't realize that you're actually still dead and will let you roam free. This results in the zero health as you're dead and the invincibility as you can't kill something that is already dead. You won't be able to regain any health or blood. You're weapons are now invisible and I am not really sure why. Your vampiric sense only lasts for half a second which will effectively softlock the game if you need to eavesdrop to continue, ex. when you are about to find Nurse Crane. Although you can spam the vampire sense button and still be able to see the hearts as if you were holding the button. The lock on button will do nothing which limits the use of many abilities including the ultimate. Hitting a blocking enemy will force the player to be stuck in place until they dodge. The player experiences no form of knockback or hitstun by enemies which makes it look like the enemies swing right through the player as if they were a ghost or hologram.
Luckily, there are two ways to fix this glitch.
Rest. Simple and easy but not preffered by many players who may want to preserve district health
Get command grabbed--they instakill you now. This means if there is a skall, let it attack you until it jumps on you and sucks your blood--you'll die instantly. This also works for Brawlers, with their knife charge, and any bosses that have a command grab like Fergal. That means if you're trapped in the Mary Reid boss fight with this, you literally have no way to die until you kill her with the unfair advantage.
That's the gist of it. Whether you want to abuse this glitch or just want to fix it, this thread may help you out. Also hoping the devs can see this so they can see exactly how to replicate it and what side effects it has. Hope this helps out any community managers, devs or wandering immortals.