Failed to open a session for the virtual machine

[user07]

I have installed Ubuntu on Virtual box which was working fine for past 2-3 months but recently i powered off the machine and ubuntu aborted.It is not opening now ans giving following error.:

Failed to open a session for the virtual machine

No error info.

Result Code: RPC_S_SERVER_UNAVAILABLE 0x800706BA (0x800706BA)
Component: ProgressProxy
Interface: IProgress {c20238e4-3221-4d3f-8891-81ce92d9f913}

Please help