Tenant didn't pay rent, claims I changed locks

[Al Pat]

I have a tenant who has been with me for last six years. She has always been very punctual about paying on time and has never complained. Zero maintenance and well maintained. I didn't get her rent by 5th so I texted her if she is going be late and she responded saying I assumed she was going to be late and locked her out. She couldn't unlock the door last night and assumed I changed locks. Well, that was news to me because I was out on vacation for the last eight days. So when I returned home I went to check and both locks were functional, though I didn't open the door because I didn't want to trigger the security system. I have been calling her and texting her all day but she is totally nonreponsive. I can't even explain her anything. I told her that I never would change lock for any tenant even though they are late because it is not legal and also stated that she is one of our best tenant so it's never an option. Also informed her that dead bolt lock may have brock and need her permission to have locksmith enter the premises but still no response from her. What should I do?