It looks there may be some incompatibility with the kernel after the reboot; it doesn’t sound like you had changed your kernel of your droplet, but I haven’t found that Digital Ocean feature to change kernels to work all that well, partly because changing kernels sometimes means changing the OS. I would say that your best bet is to see about restoring from backup if you have one.

Otherwise, you can try going into single user, recovery mode. In the grub boot menu, there may be an option there for a recovery mode, or you may be able to edit the kernel options with the parameters init=/bin/bash to drop into the bash shell. From there, you may be able to execute journalctl -b and hopefully gain more information there on why the boot failed and possibly try some repair work.