my cookie dough was in the fridge overnight and now its rock hard can I microwave it?
I'm going with a tentative "yes, but" here.

First let's have a look at what happens in your dough and why most recipes recommend refrigerating the dough before baking.

A "basic" cookie dough consists of fat, flour and sugar (plus a few extras, obviously) that form a delicious, crumbly substance after baking. The secret to the way the cookie crumbles (pun intended) lies in the way the fat separates the flour and any kind of liquid involved to inhibit gluten formation - which would translate to a tough and rubbery cookie. But the fat must remain in small, yet coherent pieces (even if we don't see them in the cookie dough), which will melt during baking and create tiny "shards" or the delicious buttery crumbs of our cookie. A very extreme example of the mechanism is puff pastry: layers of dough separated by thin layers of fat.

A dough with lots of fat will be soft at room temperature (perhaps even too soft to work with) and firm when cold. So when a dough is too firm, letting it warm up a bit is the obvious and correct choice.

But if you use your microwave, you could easily warm it up too much - making the dough hard to handle and you might have to cool it again. There is another issue if you warm your dough just a bit more: Did you ever heat butter in the microwave? It melts quickly and it does so unevenly. Some parts are still soft, others runny. This can easily happen to the fat in your dough as well. But if you get liquid fat, it can't fulfil the "form pockets and later crumbs" requirement stated above, affecting the cookie texture negatively.

So my conclusion:  


The safest method is to take the too-firm dough out of the refrigerator and simply wait until it's soft enough. If you want to speed the process up a bit, choose a warm spot. Shaping the dough into a disk instead of into a ball before refrigerating will help the dough warm up more consistently, but not all doughs are firm enough to be shaped (but I suspect it's possible in your case).
If you absolutely must use the microwave, do so very carefully, on the lowest setting and in short intervals to prevent the fat from melting.