the timer module can fail after years of use. there's no user replaceable part inside.
you can try to bypass by flipping up the cover and pull down the switch from Auto to I. if doesn't work, you can try to swop the timer module with the one on the right side. you need to open the DB cover and then disconnect the wires connected to the timers. remember to turn off the breakers first and confirm there's no electricity flowing by using a test pen. once the wires are disconnected, make sure the wires do not touch anything as you can cause a short.
if confirmed timer failed, you can buy a new one online or from electrical component suppliers.