VirtualBox Error after scanning for error on host HDD

[chipit24]

My computer has been running a bit slowly recently so I decided to run CHKDSK on my host OS. This scanned for and fixed errors on my HDD. I then defragged my HDD. But now, when I open up VB I am greeted with the following error:

One or more virtual hard disks, CD/DVD or floppy disk image files are not currently accessible. As a result, you will not be able to operate virtual machines that use these files until they become accessible later ...

And when I try to run my VM (I only have one - which is an Ubuntu 14.04 LTS server), I get this error:

Failed to open a session for the virtual machine Ubuntu server.

No error info.

Result Code: E_FAIL (0x80004005)
Component: ProgressProxy
Interface: IProgress {c20238e4-3221-4d3f-8891-81ce92d9f913}

When I try to do a full clone of the VM I get this error:

Failed to clone the virtual machine Ubuntu server.

Could not open the medium 'C:\[...]\Ubuntu server.vdi'.

VDI: invalid header in 'C:\[...]\Ubuntu server.vdi' (VERR_VD_VDI_INVALID_HEADER).

VD: error VERR_VD_VDI_INVALID_HEADER opening image file 'C:\[...]\Ubuntu server.vdi' (VERR_VD_VDI_INVALID_HEADER).

Details
Failed to clone the virtual machine Ubuntu server.

Could not open the medium 'C:\[...]\Ubuntu server.vdi'.

VDI: invalid header in 'C:\[...]\Ubuntu server.vdi' (VERR_VD_VDI_INVALID_HEADER).

VD: error VERR_VD_VDI_INVALID_HEADER opening image file 'C:\[...]\Ubuntu server.vdi' (VERR_VD_VDI_INVALID_HEADER).

Details
Result Code: E_FAIL (0x80004005)
Component: Medium
Interface: IMedium {05f2bbb6-a3a6-4fb9-9b49-6d0dda7142ac}

When I create a new profile using the same VDI image, I get the same errors.

What do these errors mean, and is there some way to solve it?

Additional information

VirtualBox version: VirtualBox Graphical User Interface Version 4.3.12 r93733
Host OS: Windows 8.1 64-bit (8 GB memory)
Guest OS: Ubuntu Server 14.04 LTS 64-bit (1 GB memory, guest additional installed)
Guest log file: Attached

[mpack]

The latter error message is clearly telling you that the VDI file header has been corrupted. Probably you had a doubly linked cluster or suchlike which chkdsk "corrected". These are host OS and hardware issues and have nothing to do with VirtualBox.

You can try opening the VDI using CloneVDI. If this also refuses to open the VDI then you definitely have corruption.

The only sure fix for a corrupted VDI is to restore it from a backup, ideally a recent one. When was your last backup taken? (says he, innocently).

Is the data important? If not then another quick fix is to delete the VM and create a new one.

[loukingjr]

(says he, innocently).
.