There are many specialist lawyers out there who do a "no win no fee" case for you.
Most I understand do a free interview first to decide whether or not they think you have a case as if you don't there's no point in them taking your case on, this at least will give you some idea if the legal profession believes you have the makings of a case.
I guess negligence is the big one as they hold all the evidence it might be difficult to prove in a way.
Generally I'm against suing the NHS but where they have clearly failed in their duty of care to the detriment of someone's health, it sometimes is the only way to make things improve though.
Obviously I don't expect you to put all the circumstances on here so I suggest you get some outside advise that you do have a case.
The free interview and advise should come without you having to commit to anything so you could start there.
K