Turin,
George has it right - if the aircraft can't get into the 'powered lift' mode, the only options would be divert ashore for a conventional rolling landing or, if doing full blue water ops (no short diversions) it's going to be the ejection seat route and ditch the aircraft. There's no barrier system on the UK QE class or the USN LHDs.
A cat and trap aircraft has the barrier option on the carrier if it has problems with its own systems - but it can't take the barrier if it can't get down to normal 'trap speed'.
Of course, if a land based aircraft is badly damaged, ejection is often a safer option than trying to get it down on the ground and risking the pilot.
As an aside, I got to see some of the reports from the STOVL technology studies. Vulnerability was a potential issue, but the bigger problem was the volumetric impact of the gas ducting on the packaging of the rest of the aircraft systems. Early on, it was realised that designing a stealthy STOVL aircraft (which forces many systems that are located in external pods on legacy aircraft inside the outer mould line) would lead to a densely packed airframe. Gas ducting made an already tough challenge nearly impossible.
Difficult stuff, this powered lift.
Best regards as ever to the really clever people who take this challenge on.
Engines