Mutual Separation Agreement - Severance - Signed & Notarized - Now they want to Void & Null it

I was let go by my employer after 6 years of reliable service. They offered me a mutual separation agreement with severance included in the agreement.

I got it signed and notarized and employer got it signed and notarized as well.

Now they are saying that they want my help and to do phone calls and online meetings to help them out. They say my payment of severance is contigent upon me completing what they want on their terms in order to get my severance. The agreement does not state that anywhere at all. I also had unused vacation days to boot.

Now since they say I am not doing what they want, they are going to make the arleady signed agreement null and void. And they want to draft a new agreement to sign.

What can I do in this regard? They keep emailing me and calling and saying if I do not respond in a day the agreement will be null and void.

They let me go yet still want my help and my help without additional compensation for such.

Please help. What can I do?