I am using VirtualBox 4.1.18 and had a virtual machine running Windows 8 RC with a single VHD, which was initially sized at 25GB (too small!).

After installing the OS and some applications I ran out of disk space so shut down the guest and then used this command to resize the VHD to 80GB:

C:\Program Files\Oracle\VirtualBox> .\VBoxManage.exe modifyhd "D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vhd" --resize 81920
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
C:\Program Files\Oracle\VirtualBox> .\VBoxManage.exe showhdinfo "D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vhd"
UUID:                 03fb26e7-d8bb-49b5-8cc2-1dc350750e64
Accessible:           yes
Logical size:         81920 MBytes
Current size on disk: 24954 MBytes
Type:                 normal (base)
Storage format:       VHD
Format variant:       dynamic default
In use by VMs:        Windows 8 RC (UUID: a6e6aa57-2d3a-421b-8042-7aae566e3e0b)
Location:             D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vhd

So far so good. However, when I started the guest up again I got the dreaded:

Fatal: No bootable medium found! system halted

If I boot into GParted it shows a single 80GB drive as "unallocated". The option to scan for and attempt to repair a filesystem doesn't find anything.

I also tried cloning the VHD into a VDI file, just in case that magically fixed it:

C:\Program Files\Oracle\VirtualBox> .\VBoxManage.exe clonehd "D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vhd" "D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vdi" --format VDI
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Clone hard disk created in format 'VDI'. UUID: baf0c2c4-362f-4f6c-846a-37bb1ffc027b
C:\Program Files\Oracle\VirtualBox> .\VBoxManage.exe showhdinfo "D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vdi"
UUID:                 baf0c2c4-362f-4f6c-846a-37bb1ffc027b
Accessible:           yes
Logical size:         81920 MBytes
Current size on disk: 24798 MBytes
Type:                 normal (base)
Storage format:       VDI
Format variant:       dynamic default
In use by VMs:        Windows 8 RC (UUID: a6e6aa57-2d3a-421b-8042-7aae566e3e0b)
Location:             D:\VirtualBox VMs\Windows 8 RC\Windows 8 RC.vdi

Is there anything else I could try to recover the drive?

No, I don't have a backup :(

My host OS is Windows 7 64-bit.