I did this a while back with my 12 x 18. I was able to fix mine using an iron, just put a thick towel down on top, make sure you have your iron set at its highest and use a lot of steam. Iron the towel and keep ironing until you can feel the mat is straight and then pile heavy books on it and leave it alone until the next day. As tempted as you might be to 'check' on it, don't - leave it alone until the next day. I made the mistake of doing that and I had to do it again. If it's still not completely straight the following day, do it again. Sometimes it takes more than one pass with the iron.
My mat is still flat, though I have some teeny tiny warps on the edge. I don't use it often anymore, just to do straight cuts on small pieces of fabric, but if the warp isn't that big, you shouldn't have any problems resolving it.
I was actually so impressed by the way mine came out that I used the same method on a big strip ruler that I have, that one was tossed in an extremely hot room one summer and when I got it out, it was warped. I did the exact same thing on the ruler and was able to remove the warp.
Try it to see if it will work before you commit to buying a new one, cutting mats, in my opinion, are just too expensive to give up on them. The steamy iron works, you've just got to be patient.