The glass cook tops are made of tempered glass. Even small crack can eventually lead to shattering if the glass if heated in a condition where it has a weakness. In addition, depending on the severity of the crack, any liquids that boil over while cooking can end up directly on the heating elements. This could lead to catastrophic failure of the elements and/or an electrical shock hazard. It is strongly recommended that the cooktop is replaced as soon as possible.
If you need guidance on how to replace a cooktop, the following link explains:
http://www.fixya.com/support/r4249512-replacing_glass_cook_top
Repacement parts can generally be purchased on line at any of the following websites:
searspartsdirect.com
pcappliancerepair.com
appliancepartspros.com
repairclinic.com
All these sites offer great service with competitive pricing, so shop and compare. The first three sites include helpful exploded view parts illustrations to assist you with locating and properly identifuing any parts you may need.
I found the following part number(s) : WB62T10475, WB62T10476, WB62T10477
The correct part number and price will depend on your COMPLETE MODEL NUMBER and COLOR of your range.
If you have any further questions, please post back and let me know. I hope this helps you.