Talk to them in person. Something may have gone very wrong and they may be afraid to answer calls/messages.

Oh yes. Fortunately, among several instances, the troublesome tenant(s) left before I needed to go through legal proceedings.

It's difficult to keep tabs on people from a distance. First - your policy should have been clearly laid out in the lease (hoping there is one). Next, you need to be familiar with the eviction laws in your particular state, as they can differ greatly, depending on where the rental is.

Happily sold the quad a few years ago !

You might find this helpful : www.mrlandlord.com

Its your house; change the locks. if they want it badly they'll pay up you'll lose less the quicker you do it but I don't know the law in your country .

You should go and talk with your tenant if that is possible. If you can, try to get some money and a schedule for repayment
I had to do this in the past. It sucked but it was effective.
Alays get credit report or several references.

Yeah I've went through the whole landlord routine it's a pain in the butt if you really want to insist on renting a place out use a rental agency you have to give up some of your money but at least then everything is in their court and they deal with all the problems

Do it now. I've had several tenants I've had to evict. One couple left most of their belongings and after threatening to donate to Goodwill they finally came with a UHaul one day and still ended up leaving a refrigerator for my next tenants, furniture and tons of trash.

Check your state landlord/tenant laws.

In Washington State, I was required to give tenants a written, 90-day eviction notice.

For a year after their eviction (by police), I had to store former tenant's abandoned belongings.

Catching their traumatized cats was difficult. I took them to the pound.

Then cleaned up the mess, re-painted and repaired the apartment, had carpets professionally cleaned, etc.

Nightmare. I felt relieved to stop being a landlord.