For the Serial interface, sometimes hard coding the speed to 115200/9600 fixed my issue (While using Putty and the COM interface). Sometimes it doesn't negotiate the speed properly. Also, stupid suggestion, make sure you're using the correct COM port on your computer. I've done that before.
Does your security subnet have a VLAN interface on the switch? Unplug a device from security vlan subnet, and try to access it via that. Otherwise, if you have a management subnet and you have it trunked.. You could try an untagged management interface on a different switch and attempt to access it that way.
Like if you have Switch -> SG500 (Trunked for Management, Access, and Security VLANs)
On the switch before it, make a untagged port whatever your management VLAN is, and you should in theory be able to talk to it.