Was the cobra alive when dinosaurs were alive?

Cobras and other modern snakes were represented by their ancestors that may have had legs and more closely resembled lizards. Those that were adapted to living underground eventually found limb reduction, scales replacing eyelids and thinner bodies to be an advantage for burrowing. By living underground they would be able to avoid predation from and competition with the dinosaurs who were certainly sharing the ecosystem.

Current theory points to global catastrophic climate change, possibly resulting from a meteoric impact, to be the cause of the mass extinction of the dinosaurs some 65 million years ago. Those animals that would have survived to re-populate the earth would have been presented with open niches and no competition. Ancestral snakes would have returned to the surface from their underground tunnels to find ample opportunity to evolve into modern snake forms such as the cobra.

So to answer your question, there were no cobras around with the dinosaurs, but the ancestors of the cobras were around toward the end of the dinosaurs' tenure. The first cobras evolved around 25 million years ago.