Heating and A/C question?

I have a 2 story house I bought 11 yrs ago. Heat and A/C was replaced approx 13 years ago. The previous owners told me the heat and air upstairs has a 'regulator'. There is a separate thermostat upstairs that I can control heat and air with, or turn it off. I usually keep it set at the same temp as downstairs so the house maintains even heat and cooling.

I noticed a couple of days ago I have no a/c upstairs because the

'regulator' isn't working. I don't know what a regulator is in the first place.

My question is, can someone tell me what it is and if so, will it be expensive to replace?

Thanks!