I had a similar problem after making a flanel quilt.
My mat is an Omnigrid.
First I used the edge of my 6 1/2" squaring ruler to "scrape" the edges, then I wiped it down with a damp cloth. I still had some fairly good "divots" in it so I soaked a bunch of kitchen towels and laid them flat on the mat pressing the water into the mat ... and left them there for a few hours or so.
When I was done I wouldn't "good as new" but it was an improvement.
Hope this helps.