i think what might be confusing people is you said you rebooted which suggests you were able to get in. So when you say you rebooted, do you mean you were able to get in and do a reboot or did you mean you cut the power then turned it back on?
Also just speculating now but how long have you had the device and are you running off an sdcard? Like David suggested, SDCards have a limited lifespan when it comes to writes and HA is pretty rough on them if you let it record every bit of history for every entity. If you search the forums for ‘sdcard dead’ you’ll find a common trend of sdcards turned read-only or becoming corrupted after a while.
If you are using an sdcard and you are locked out your next best bet is probably to shut down the pi, take out the sdcard and go inspect it on another machine that can read linux partitions. From there you should be able to use available tools to see if its become read only or otherwise corrupted. If so then you’ll have to migrate to a new sdcard.
Or if that’s not it you should be at least able to see what is consuming the space on the drive. Logs are another good place to look, there are a number of reports of out of control logs then causing system lockup. If you can free up some of the space on another machine then you should be able to get back in and use some of other suggested debugging techniques to isolate and fix the root cause.