Im sorry I offended you but you didn't respond when your tenants reported the first problem and it appears from my perspective you are only responding now as they don't have the use of thier plumbing at all now. " A few months ago the tenants in the basement said that the toilet wouldn't flush when the washing machine was being used. I told them to let me know how it was going and never heard anything about it until today." You told them to let you know how it was doing. They did. They can't use toilet when laundry running. That is a big red flag and as the landlord it's your responsibility to know that and deal with it in a timely manner. If you don't know enough about plumbing to recognize the possible severity of the situation, you might want to hire a qualified person to oversee your properties
"I buy them movie passes and lots of other gifts to thank them for being such great tenants and I am always fast to fix everything. I realize that rental properties cost money. I was simply asking this question on here to see if there was an easy fix that they could do to avoid paying a plumber. That's just being financially responsible. " No its being cheap. If you had called a plumber and then come here for advice, suggestion etc then that would be being financially responsible.
Buying movie passes and shiny bobbles for your tenants might make you a good friend but it does not make you a good landlord. Providing the services your tenants pay you for does.
Again, your motivation is all too clear here in your statement from before.
"I'm petrified that this is going to be a big expensive issue costing hundreds of dollars."
I hope I'm wrong and that it's just a communication error or something but I have been in the industry, plumbing and the rental income property, for far too long to have top much faith
I'm glad a plumber is on the way and I truly hope it is a simple fix.