Writing to a professor who contributed to my research paper
Just say it as you have here, I think. It seems perfectly reasonable. It also seems like a good idea to reconnect for future projects. 

But you don't say why you were contacted. Either you were listed as a co-author or were given an acknowledgement. Some journals get permission for all acknowledgements from all parties. It would be good to know beforehand which case it is, and what your feelings are about how the professor included you. 

But you gain nothing, now or in the future, by staying silent.