The first thing is to stay calm - not easy to do in the circumstances, but it will help your case no end if you can manage to explain your problem clearly and calmly to Dabs.
From what you have told us, your monitor has most likely been stolen. Probably by someone posing as yourself and signing for the monitor.
As dabs are confident that it has been signed for, it should be straightforward for that signature to be compared with yours, and for it to be clear that you (or your representative) have not signed for them
Your contract is with Dabs, not Parcelforce. Get back on to them, initially by phone. Keep a written record of conversations (time, date, name of contact etc), and confirm it in writing to Dabs at the earliest opportunity.
Dabs will obviously have to carry out further checks with Parcelforce, to satisfy themselves that you are not trying to defraud them, so it may (sadly) take a little longer to resolve.
You might like to suggest that they sell you another monitor, and refund the cost as soon as they are satisfied that you did not receive the first one - this will at least get you a monitor sooner.
If you do not get satisfaction from dabs fairly swiftly, I suggest contacting your credit card company. They will be very interested in a deduction being made from your account without goods being received.