What do you do when the maintenance required light stays on?

Most auto parts stores (AutoZone in the US) will read your codes for free, and attempt (sometimes successfully) to decipher them. They may not reset your Check Engine Light, but if it was a temporary condition, disconnect your battery for about a minute and reconnect it. This will reset the ECU.

If the condition persists, the Check Engine Light will come back on.
You can unhook the neg battery terminal for 5 minutes or pull the main fuse for the same duration. Either will clear the codes and reset the monitors. The thing is though, even if you erase a code, if it is recurrent, it will set the MIL light again. And when you reset the computer, it takes a series of stops, starts, cold and hot cycles for the car to relearn settings again. This relearning will prevent you from getting the vehicle to pass inspection, unless you hit the sweet spot when the majority of the monitors are reset AND before the MIL light illuminates again. Generally when I receive codes, I make note of them, and reset the memory and see if they return. Sometimes they do not.

He is asking about the Maintenance Required Light and not the Check Engine Light.