Are you talking about the PATS immobilizer?
Took me a second due to the vagueness of your post (IE What is flashing, what car you have, is it US/UK spec, ect.), but if you're talking about the light in the center of the dash flashing 15 times then that means that the key is not recognized by the immobilizer (Transponder key not recognized by PATS).
I don't know how to fix this, I've only encountered this issue once when I tried starting it with an un-programmed key LOL...
Hopefully someone on here has more info for you... Only thing I can tell you to do is try another key and check the fuses while you wait for someone more experienced with PATS.