So I have a 401k at the company I currently work for. Which I will be leaving pretty soon. I have a pretty nice (to me) lump of money in that 401k, that I am now terrified is going to disappear into the ether when I leave the company.

I am very dumb about money/banking things and have no idea how this works. Can someone explain to me how not to lose that money? I just have no idea how to approach getting it moved to a place where I have access to it, who to call, etc.