EDIT: Looks like I used to manage your site. I’m rebuilding it now. You also needed to do the PostgreSQL 15 update; I did that too.
cd /var/discourse
./launcher rebuild app
That should fix it. It’ll take about 10 minutes. If not, see if the disk is full or you need swap.
df -h
free -h
There’s a chance that you need to add swap.
If that fails you want to solve the problem with money, contact me and I’ll get you back up and running.