AC is not working

My wall HVAC thermostat was loose on wall, so I made it tighter, but it was reported by my tenant that heater is working but AC is not working (not even kicking off). The AC breaker is on.

One thing I found in the attached picture which was taken before I make it tight: there was U pin running between R and RC, which is now missing after I reinstalled.

Is that U pin or jumper wire missing not to kick off the AC? I think I have lost it :(. where to get that from?

Please advise.