Your website broke after a PHP upgrade. What should happen next?
Keep the error details and ask the host what changed. I'd want a backup and a test copy before anyone starts changing several things at once.
Record the change before doing anything else
A blank page after a server change is a symptom. It does not tell you which theme, plugin or piece of custom code has failed. Ask the host for the previous and new PHP versions and the relevant error log.
Note when the problem started and what still works. The homepage may load while a form, checkout or background task fails. That information helps decide which checks belong in the repair. Avoid posting credentials or a full private log in a public forum.
Ask whether a temporary rollback is possible
The host may be able to restore the previous PHP setting while the fault is assessed. That can buy time, but older releases may be unsupported or unavailable on the new platform.
I would treat a rollback as a temporary service-restoration decision. It does not repair the incompatible code. Agree who will investigate the cause and when the next change will be tested.
Protect the working information
Keep a backup of the database as well as the files. For WordPress, those are separate parts of the site. Check that somebody knows how to restore them before relying on the backup.
A shop or booking site needs particular care. Restoring an old database can also remove orders or appointments created after that copy was taken. Tell the person repairing it what new information must be preserved.
Test the dependencies on a copy
Check the platform, theme, plugins and custom code against the proposed PHP version. Updating a dependency may be enough; replacing abandoned software or repairing custom code can take longer.
Choose a currently supported PHP version that the application can use. The official PHP support page gives the dates for each release. I would check that at the time of the work instead of choosing a version because another website happens to use it.
Agree the tests that mean the job is finished
List the business functions to check before the live switch: logging in, sending an enquiry, taking a booking, uploading a file or processing an order. Add scheduled work and external connections where the site uses them.
If an upgrade would require extensive repairs, compare that quote with a rebuild. I'll explain the work behind each option so you can decide how much to invest in the old site.