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Hello and welcome to just answer. I am sorry to hear you are having trouble with your thermostat. There shouldn't be any period it would need to wait. Are you sure the batteries are new and good? May want to try swapping them out with another pair just in case.
You did not disconnect any wires or anything when pulling the unit off did you?
If that does not seem to be the problem I want you to try turning off the the circuit breaker that goes to it to see if we can try restarting it. If that does not work unfortunately your unit display may be going out on your but Honeywell typically has a 5 year warranty on these so you could always contact them and have them send you out a new one or someone to come look at it.
I was wondering if any of my advice was of any help to you? If not please just let me know instead of rating negatively and I will opt out and open this up to other experts for other ideas.