First make sure the app is closed in multitasking then try again. Double tap home > Hold down a icon and click minus to close. Since there is limited information im just going to give you all I can think of right now. Trying doing a "Hard Reset" to do this hold down the Home button and Sleep button at the same time until the phone shuts off, then turn it back on and try the app. If that does not work try and go into the safari settings and clear the data (Settings > Safari > Clear Cookies and Data) and once again try to open it. You can also try to restore your phone from a backup on iCloud/iTunes, or simply just download another browser like Chrome.