Unable to restore snapshot-based AMI to m1.medium instance - says that partition can't boot

I have an old but still running well m1.medium instance based on Ubuntu 18. I've received notice that the instance needs to be stopped and restarted as a result of degraded hardware. I've gone through the steps of creating a snapshot of the EBS volume, turning it into an AMI and launching an instance from it. The instance does not boot - taking a screenshot shows that it is stuck a the bios screen and the machine considers there is no bootable partition. I've tried a few options for creating the AMI such as picking gp2/gp3 or paravirtualisation or hardware assisted virtualisation, but the results are similar.

I've tried going back to the previously working AMI that I installed the system from, and this will not boot either. I'm therefore concerned that if I follow the instructions and stop and restart the instance, that it will not start again. I have two weeks before the deadline where the instance will be retired. What can I do to troubleshoot and resolve this issue?