Cracked screen is not covered by Apple's limited warranty.
What do you mean "appeared"? Cracks generaly don't "appear", they are caused by external factors.
This Warranty does not apply: (a) to consumable parts, such as batteries or protective coatings that are designed to diminish over time, unless failure has occurred due to a defect in materials or workmanship; (b) to cosmetic damage, including but not limited to scratches, dents and broken plastic on ports; (c) to damage caused by use with another product; (d) to damage caused by accident, abuse, misuse, liquid contact, fire, earthquake or other external cause; (e) to damage caused by operating the Apple Product outside Apple’s published guidelines; (f) to damage caused by service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (“AASP”); (g) to an Apple Product that has been modified to alter functionality or capability without the written permission of Apple; (h) to defects caused by normal wear and tear or otherwise due to the normal aging of the Apple Product, or if any serial number has been removed or defaced from the Apple Product.
Under 99.99 percent of the circumstances iPad screen do not crack spontaneously. Accidental damage is not covered under the standard warranty. AppleCare + does allow for very low cost replacement but you have to have purchased that previously.
Best bet is to make an appointment at an Apple store for a consultation.
Apple's Limited Warranty http://www.apple.com/legal/warranty/ for iPad excludes coverage for damage resulting from accident, disassembly, unauthorized service and unauthorized modifications.
If you own an iPad that is ineligible for warranty service but is eligible for Out-of-Warranty (OOW) Service, Apple will replace your iPad with an iPad that is new or equivalent to new in both performance and reliability for the Out-of-Warranty Service fee listed below.
iPad model Out-of-Warranty Service Fee iPad mini $219 iPad 3rd, 4th generation $299 iPad 2, iPad $249
A $6.95 shipping fee will be added if service is arranged through Apple and requires shipping. All fees are in US dollars and are subject to local tax.
Certain damage is ineligible for out-of-warranty service, including catastrophic damage, such as the device separating into multiple pieces, and inoperability caused by unauthorized modifications. However, an iPad that has failed due to contact with liquid may be eligible for out-of-warranty service. See http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=ipad
Make a Genius Bar Reservation
You can get the glass replaced at 3rd party repair sources for less $, however, the Apple warranty will be voided.
iPad Repair & Screen Replacement Services
RepairZoom iPad Repair
Unfortunately, unless you have video of it cracking itself the presumption is that it cracked because of something happening. Your best bet is to do as was suggested above, take it to an apple store (and best to make an appointment) and talk to the people there. They have the most latitude in handling situations like this.