Cannot open bonnet fiesta tag snapped off

Can you see the end of the cable through the front grille? On some vehicles you can use a extra long flat screwdriver to push the release through the front grille.
Some S3 and A3 models have a small access hole between the bonnet and grille. A thin wire hook and a lot of poking around can release the bonnet.
if this doesn't work you might have to destroy the front grille to get access.

Use a long screwdriver to pry the catch,
Access through the grill or from under the front of the car

do you know were the cable has snapped front or cab ?
cab you need to use a pair of plyers, front snapped.bit more tricky. and its going to cost you a bit more to just get to the cable and you will need to replace parts on the front.

get assistant to push down on bonnet whilst you pull the lever ,then once you pull back the lever and hold it back they try to lift it so it clicks up a bit then you open it by the safety catch

if the cable has snapped then you will need a very long screwdriver/rod to go from underneath the car to the release catch,it is a nasty job that will give you "hours of fun",you unscrew the bonnet adjuster by the way.

You don't specify whether its just that you can't find the release catch, or whether the release catch is stuck / broken,

In the UK (Right hand drive car) the release catch is situated in the left (passenger side) footwell, within about a foot of the lower front door hinge. In a Left Hand Drive car, I guess this would be reversed (i.e. right hand side footwell). Pull, and it semi releases the hood. Next, walk to the front of the car, pull the hood up about 1" and insert your fingers about a foot left of center. Sweep to the right, and you'll release the secondary catch and the hood will be open.

However, if its stuck / broken, then ......

You need to work underneath the front of the vehicle - the idea is to get at the catch and release it with a long screwdriver flicking the catch to open the bonnet -or getting a grip of the cable if it is hanging loose ( i.e. broken ) then getting a grip of the cable with mole grips or even pliers - to pull and release the cable / bonnet.

Once the bonnet is open - you can deal with renewing the cable assembly, or greasing it thoroughly.

Lube the cable and work it back and forth. Also it could've been in an accident and not properly aligned.

Hopefully the cable has not snapped. Have someone else lean on the bonnet a little. (Not too much or you will dent it) Try and operate the catch while they are leaning and releasing. Or sometimes dust can accumulate on the catch. Perhaps you can spray under the bonnet with a can of WD40 and hit the catch. Try the above trick again after you have sprayed it. Once you get it open find the cause before you close it again :)

Most of the time the cable snaps at the release handle. If you remove the handle and get the end of the cable, you can cut off the cable jacket and use needle nose pliers or vise grips to grab the cable and pull to release bonnet.

There are two release catches about 15" to the right and left of the centre safety catch. Roughly at each end of the top of the radiator.

The bonnet release cable goes first to the right hand one then there is a wire from that to the left, in a daisy chain way. They are typically French, cables fit into plastic levers ( a bit like bike brakes) not metal which break easily. replacements are not cheap. You need to be able to pull the cable to both catches to get the bonnet up. I managed to do this from underneath. In my case the, left hand one was broken and right hand one was working, but the getting it open broke the bit that that pulls the left. Managed to away with buying just one new catch as they are identical, swopping the right partly broken one over to the left side.

HTHs