All my reminders have disappeared after upgrading to iOS 13. How can I recover?

You can't. As the warning noted when you converted your reminders in iOS 13, if you chose to convert them, they won't be available on any other devices that cannot use the new format.

The new format will be supported in Catalina when it is released in October.

You can view them in a web browser on iCloud.com.

Get ready for the new Reminders app - Apple Support

can you please tell me how I can view my old reminders on iCloud ..... all are very important, B'days, important dates etc ..... **** !!!!

Log into iCloud.com with your AppleID. Click the Reminders button.

It is only getting worse. Also your calendar will empty... I am in contact with Apple support, but no solution so far.

I don't believe anything about Calendar was changed. Were your calendars all in the cloud, or did you have some locally stored ones?

If all in iCloud, disable Calendar in iCloud System Prefs, then re-enable it.

Go to the Edit menu and see if there's an "Undo XXXXXX" option.

There was on why iPhone which let me restore all of the reminders that went missing after the upgrade.

can you please tell me how I can view my old reminders on iCloud ..... all are very important, B'days, important dates etc ..... **** !!!!