Does Apple repair iphones on Virgin Mobile network?

Apple will handle your iPhone regardless of where you purchased it. Apple does not repair iPhones, but will replace the entire iPhone for a flat fee. See the Service Pricing section here:

http://www.apple.com/support/iphone/service/faq/

There are independent iPhone repair services that may be able to do the repair for less than Apple's out-of-warranty charge. Do a web search for "iPhone repair". Note, though, that if you elect to have the iPhone serviced by an unauthorized shop, all further warranty and support from Apple will be voided.

Regards.

Yes, Apple Store can help you. If there is AppleCare + plus, the Replacement iPhone 4S will be $50, if no Plus, the perfect Replacement iPhone 4S exactly like the one you have, will cost $199. Make Genius reservation so you do not have long wait at the store.