If you are a grandparent that has been denied access to your grandchild or grandchildren, it will certainly be understandable that you are feeling let down and agitated, but the problem still remains that you might not by law technically have any rights to see your grandkid/grandkids at all.
(The content here isn't in any form a 'replacement' for gaining legal advice from a qualified attorney - any questions should be directed at them over grandparents rights etc.).
However, there are perhaps a few steps that you can take to improve the chances of you being able to see and being allowed to form a relationship with you grandkids, but it is important to bear in mind at the end of the day it is the guardians of the child that have all the rights, as to who they let their kids see.
A positive step that you can take, is trying to sort out the conflicts and disharmony with your kids (the parents of the grandkids) or their respective guardians and develop some sort of understanding of what has gone wrong and try to deal with issues that have been conjured up. Have a real deep discussion over the issues that have arisen and see what routes can be taken in order to help build (or rebuild) and strengthen your relationship with the parents of your grandchildren.
Be sure to approach this in the right way and with a lot of care however, don't feel like you are pressurizing your children to make peace, just get in touch with them and state that you are simply wanting to mend the rifts, leave them with the decision or not to get back in touch with you, via the communication method they see fit.
You need to do what is best for the grandkids and if you feel that you can contribute towards their lives, then perhaps you just have to bite your lip and agree that you were in the wrong, even when deep down you feel like you weren't, the lesser of two evils. Hopefully then when things are going smoothly, the parents fo the grandchildren will then eventually feel comfortable with you to form a relationship with their children (your grandkids) and allow you to visit them.
For more advice on how to approach such a delicate issue I would suggest reading books such as the 'Grand Wishes: Advocating to Preserve the grandparent Grandchild Bond'.
If you believe that having a counsellor or a professional who has experience sorting out family issues, to see over the 'making up' process then this is something you need to first consult with the parents of the grandchildren first to see if they would agree to take part in, this does however show that you are serious about wanting to rebuild bonds. There are benefits to someone who isn't apart of the family and has an unbiased view and has skills around sensitive topics, this will then likely help proceedings go more positively and have more structure.
The only real other option you have is if you feel that the grandchildren are at risk you need to think about applying for custody of your grandchild, seeking legal advice from a qualified attorney to prohibit the parents of the child from their parent rights and then going on to adopt the grandchild and become their guardian.
For more information and advice to staying in touch with your grandchild I would get hold of books such as 'No Greater Loss: The Guide Book to Today's Grandparents Rights'.
Hopefully I have provided you with more options for a grandparent being denied access to their grandchildren, however if you are wanting to seek legal advice it is best coming from a qualified lawyer and bear in mind that this article does not substitute for legal advice on how to approach your individual issue.