Unplugged alarm panel - battery died - keypad says low battery

Well, it's either dead, and you need a new battery, or it will recharge and perhaps you don't need a new battery. Since the typical alarm is very "trickle" rather than active charge, it may take 2-3 days.

Depending on what its built in-protections for the battery are, a complete discharge might have damaged it (if no low voltage cutoff - lead-acid batteries HATE being anything like "fully discharged" - they are not NiCads) or not.

Alarm batteries are typically designed to protect in case of a power outage for a certain period of time - if the time yours lasted was particularly short, it probably needs to be replaced. You could check the actual voltage with a meter to see if it's alarming (11.99 or less; or 12.5 or less, depending who you ask. 10.5 and under are pretty clear indications that one or more cells have definitely given up the ghost) and if it rises before you check it again.

Unclear why you chose to do this - there are not very many reasons to, and several not to. Despite being nominally rechargeable, there's a rather small finite life to these batteries, and deeply discharging them eats into that.

Common recommendation is to plan on replacing on a 3-5 year cycle (how well the alarm "checks" the battery varies between alarms, and it may or may not give you any indication that the battery is useless before you have a 5 minute outage and the alarm goes belly up in 3 or 30 seconds - which can be one reason to unplug for a couple of hours once in a while, but not to the point of killing the battery unless it can't make it for a couple of hours and you replace it.)