How do I remotely reboot my computer if I have an encrypted HDD

There is no safe way around this.

Either you:

1. Enter the password automatically. Thus negating the security.
2. Or you manually enter it at boot.
3. Or you manually enter it after booting a small unencrypted partition.

The last allows you to boot a minimal system into which you can SSH and then manually mount the partitions with protected data, but it also weakens the security because people can compromise this unencrypted part and wait for you to enter the encryption password.

If your server is equipped with a remote management feature (E.g. Dells DRAC, HP ILO etc) and that is on a secure network then you can also consider using that to remotely grab the console and enter the password (A sort of remote option 2). That assumes you trust the remote management features and network.