This sort of problem with your alarm system is almost never due to a faulty control panel. Statistically, failed panels are the least likely cause of any problem in an alarm system.
From the symptom you describe, the most likely cause is a weak (or dead) control panel battery, i.e., the sealed lead-acid (SLA) battery that lives in the steel cabinet with the control panel. It's either a 12 V, 4 Ah (12 Volt , 4 Ampere-hour) or a 12V 7Ah battery. I recommend replacing whichever it is with a 7 Ah battery--they last longer and are more cost-effective in the long run. Any Batteries Plus outlet or equivalent will carry them, it's a widely-used battery.
When your alarm trips and is driving the loud siren(s), it's using more Amperes than the AC power transformer can supply, and the battery has to supply the rest. When the battery is too weak to supply the demand, the voltage drops down below what the panel needs to operate properly, and the keypad(s) won't work: The panel becomes locked in alarm mode, unable to process data from KPs to Disarm.
You probably didn't need to replace the 6128 keypad for this particular problem, but keypads are one component with moving parts (the keys) and have to be replaced eventually anyway, so it's just as well you got a new one. The 6128 was on borrowed time anyway.
For future reference, it nearly almost always helps to name the model of the control panel when you post in with a question. In this case it didn't matter--weak/dead batteries cause that symptom with almost all panels, but the majority of problems need a make and model reference so we know what we're dealing with. I know your panel is old, because the 6128 has been out of production for a long time---kudos to you for keeping it going.
