Your board is phpBB version 3.0.x, for which support ended a year ago, except for help upgrading. We can help you upgrade for free here in the support forum. Here are the instructions - https://www.phpbb.com/support/docs/en/3 ... /upgrade3/
If you backup your files and database, nothing can go wrong from which you can't recover.
I believe the errors are because your host has updated to a php version with which 3.0.x is no longer compatible.