If you were upgrading your IPB version... you most likely were upgrading to the latest version.
#1 - The new version requires at least PHP8. Are you running PHP8 or PHP8.1? (If not, you need to be to run the latest version.)
#2 - If you meet all of the system requirements, including the PHP version... you most likely have a 3rd party resource that is causing problems. You can try using recovery mode to disable everything and let you try to login with everything turned off. You can then turn on resources one by one to figure out your problem child.
When you use recovery mode... you set the specific variable in your constants.php file. (If you don't have one, you create one.) Then login to the ACP... you'll get a message saying everything is being disabled. Once done, you'll need to remove the recovery variable (or delete the constants file if that was all that was in it.) From there hopefully you'll be able to login to your ACPa again.