Why do I have "500 Server Error" on my Java appengine app after 5 years without having any problem? – PHP Versions

I started this website (http://speaklikewater.appspot.com/) using Java appengine 5 years ago. It uses some datastore files.

I had never had any problem since then.

But yesterday, I see this error "500 Server Error" when I visit my website.

Error: Server Error The server encountered an error and could not
complete your request. Please try again in 30 seconds.

I looked into the log file

It seems that I have "Billing disabled. Shutting down instance." but I have never been worrying about Billing stuff since I launched the site.

I used the free version, they control the number of queries. But the site only has a few queries a day.

But this error will be reset after 24 hours. It has been over 24 hours already but I still have this problem.

Could you tell me how to fix it?

Note: They sent me this email

Add your payment information by January 31, 2021, to continue using
your App Engine standard applications. Hi Hung,

On November 30, 2019, the App Engine standard environment began using
Cloud Build during deployments. Cloud Build, along with all Google
Cloud products, requires a valid form of payment. At that time, we
asked you to link your App Engine projects to a billing account with a
valid form of payment to continue deploying to your projects.

Starting January 31, 2021, all App Engine projects must be linked to a
billing account with a valid form of payment to continue running. App
Engine apps that are not linked to a billing account with a valid form
of payment by January 31, 2021, will be shut down. Running instances
will be terminated and no new instances will be able to start until a
payment method is added to the project.

This change does not impact existing App Engine Free Tiers. If your
usage remains within the free tier quota, you will not incur App
Engine charges. However, any usage above the free tier quotas will
result in charges. Before this change, if you exceeded the free tier
usage, your app stopped running until the daily quota reset.

What do I need to know? Requiring a payment method helps prevent fraud
and abuse, and aligns App Engine with the rest of Google Cloud. This
allows us to spend more time delivering new features and providing
support to you as your business grows on App Engine.

Projects using App Engine Flex or the following App Engine Standard
runtimes have always required a valid form of payment and are not
impacted by this change:

Python 3 Java 11 PHP 7 Go 1.11/1.12+ Ruby NodeJS What do I need to do?
To avoid disruption, follow the instructions to create a new Cloud
Billing account (if you don’t have one) and enable billing on your App
Engine projects.

If you rely on Free Tier as a cost control mechanism, we recommend you
consider the following alternatives to manage your App Engine costs:

Set the max_instances setting to 1 in app.yaml, to reduce the risk of
exceeding the Free Tier. This setting limits your app’s scaling
ability, but isn’t a hard limit, and may allow excess usage that may
increase your bill. Set a Cloud budget alert to notify you when you
are close to exceeding your budget. Manually disable your app.
However, this only prevents charges related to actively running your
app, such as instance hours, networking, etc. You will continue to be
billed for fixed costs like Cloud Storage capacity while your app is
disabled. Programmatically disable your app to limit further costs
once a certain budget threshold is met.