Electronics Repair Questions? Ask an Electronics Technician
Try turning the phone off for a few minutes, then turn it back on again.
If the messages are still missing, your best option is to recover from a recent iTunes or iCloud backup.
If you do not have a recent backup available, then you can use special recovery software to scan the phone and recover the messages: http://www.easeus.com/mobile-tool/free-iphone-data-recovery.html
Let me know if that resolves the problem, or if you need any additional assistance.