Over the Xmas period it would seem that someone deleted a computer account from AD.  This meant that the user of that PC could not log in using that PC.  This is a Windows 2008R2 forest so to restore the computer object;

Get-Adobject -filter {samaccountname -eq “pcname$”} -IncludeDeletedObjects | Restore-Adobject

The $ on the pcname is important - computer objects in AD always have $ at the end of the name as part of the samaccountname attribute.