you need to get in the boot and pull off the trim, grab it at top, one hand either side of the middle and give it a quick hard pull. then you will need to look down and you will see the mechanism. there is a little solonoid type of arrangement with a metal rod which operates the mechanism, and this can get mid way between the two positions rendering you unable to open, the boot.
this happened many years ago on my old cupra and others had the same. I 'think' you can set the boot to lock unlock with the remote, or have it set to opening it with the key manually (if i remember correctly). i think the safer option is to set it to the manual unlock position.
anyway, you can pull on the metal rod that you will see to get the mechanism to operate then you can open the boot and work on it a bit easier.
you can try using some silicon or ptfe lubrication to get the solonoid working fully again.
For mine i did this and it worked ok for a year then did it again, i did the same, but left the trim off expecting it to happen again but 3 years later it was ok.
PS some people say you can lift the cupra
badge so its at 90degrees and give the edge of it a thump towards the boot to give it a jolt. worth giving it a go, didn't work for me but some reported it did.
also found this quote on other forum:
hi,if you access boot trim from inside of car and remove the nearside handle(recess) used for lowering boot lid (one phillips screw and a bit of wrestling) you can reach inside trim and release the boot lock mechanism and the boot will pop open-i had this
problem on my 2002 tdi se the fault was dirty contacts in the plug of the c/l actuator-a new plug and some switch cleaner sorted it out
UPDATE: see here for a good thread on here, this will
help you see what you will be dealing with.
This happened to me, and I've seen a couple other people posting asking
how to fix it. There are a couple of threads with the solution but none as an actual "
guide" so anyway... The Dummies
guide to fixing a boot which doesn't lock...
www.seatcupra.net
another thread around the same issue
The only thread with really useful information on this is archived, so I can't post there, but I'm having a real pain with fixing my boot release. Trim is off and I've done the usual of lubing the mechanism. I thought this had worked, but all I'd actually done was fix the
problem temporarily by...
www.seatcupra.net