Your Signaling Line Circuit (SLC) is actually part of the computer of the fire alarm system. The main Central Processing Unit (CPU) is part of the Main Circuit Module (commonly the MCM, the Mother Board, Etc.). The Loop Controller Module (LCM) is an extension of the CPU board, so all the boards in the Fire Alarm Control Panel (FACP) can be thought of as the "fire alarm panel" even though there are several circuit boards connected to each other.
Data is used for communication between the FACP (Fire Alarm Control Panel) and the devices. This is so a lot of information can be passed between the FACP and the modules. When you disconnect the SLC from the Loop Controller Module, you are breaking apart the computer system of the Fire Alarm System; you are disconnecting the CPU of the fire alarm panel from the modules and detectors connected to the SLC.
Your question is very common; you are not alone with wondering what happened.
The issue here is that when a device goes into alarm, be it a smoke detector, pull station, waterflow switch, etc., two things go into alarm: that device that goes into alarm and the CPU of the fire alarm panel also goes into alarm.
Normally, when the Reset button is pushed, the first thing the fire alarm panel does is to reset the device on the SLC that is in alarm. The FACP is in two-way communication with the device, and if through the SLC, the device does not say to the FACP that the alarm is reset, the FACP stays in alarm.
It can be that the device still detects a fire, it can be there is a fault with the device so it thinks there is a fire when there is no fire, it can be the device is disconnected from the panel (you did that when you removed the SLC) the device will not tell the FACP that it is cleared.
The first thing you need to do before reconnecting anything is to read the panel. Usually it will tell you which device is in alarm. Use the scroll buttons or ACK buttons on the panel to read which device is in alarm. Use the camera on your cell phone to take a picture of the fire alarm panel's display or write down the description and the address (and anything else no matter how insignificant) of the device in alarm. You may need all the information later.
Go to that device and check things out. Is there water damage, is the device in false alarm, is there something wrong with anything around the device that is causing that device to go into alarm? Fix that problem first, and then reconnect the SLC.
Once all other problems are fixed, then resetting the fire alarm panel is possible.
If nothing is done to fix the problems with the device in alarm, resetting the panel may not be possible, or the alarm could come back at an inappropriate time, like three weeks from now at 3:00 AM.