You can try the following solutions when your iPhone is frozen:
# Re-charge your iPhone
# Force-close unresponsive apps if your iPhone is frozen in certain apps
# Force restart your iPhone
# Restore iPhone to factory settings using iTunes if factory reset doesn't work on iPhone
If none of the solutions works, you can try to put your iPhone into recovery mode and let iTunes help you troubleshoot.
Since the last two solutions will erase all settings and data from your iPhone, you'd better full backup your iPhone in advance. you can rely on iTunes or a third-party tool like FoneTool (quicker and easier) to make it.