If it's been a while since you've taken a math class, you'll probably be surprised at how much you've forgotten, but it won't take you more than a few weeks to be completely comfortable with it again. I hadn't taken physics since my first year of college, and when I had to study it for a new job in September, I was amazed at how much I had forgotten. It was like it all looked so familiar, and when I thought about it a bit I could remember how to do things, but it wasn't like I just knew how to do everything off the top of my head anymore. It didn't take much practice though before I got back to that point.
So if you were good at math before, you'll be fine. You might panic at the beginning because you feel like you've forgotten a lot, but it will come back to you really quickly. I don't think you actually lose problem solving skills by being out of practice (though you don't improve them either), it's just that you might forget some information, and you'll be slower at things because you haven't had to use that information in a long time.
I'm assuming from your description that you're taking something like differential equations or multivariable, something closely related to the calculus class you took before. If it's something completely unrelated like number theory or graph theory then you shouldn't be worried at all (I'm not guaranteeing you'll do well in those kinds of classes, but if you don't it won't be because you're out of practice).