My sons ex won't let me see my grandchild what can I do?

First, any question on law needs to include your general locale as laws vary by area.

Second, we need to know the background. Was your son married? Did they divorce? Does he have custody and visitation. Why won't she let you see them?

We really can't tell you what your options are without more info. The only answer we can give with the little info you told us is to consult an attorney.

Also we need to know the status of your son. Is he incarcerated or has he ppassed away ?

Although some states pay lip service to grandparental visitation rights, the bottom line is that, if the son is exercising visitation, OP can see them when the son does. If the son is out of the picture (incarcerated, dead, etc.), OP's best bet is probably to be very nice to the mother.