I'm possibly (probably) being a complete eejit here.

I wiped my ER-380M to start afresh as I had far too many old barcodes stored. In the process, my Honeywell Eclipse MS5145 barcode scanner seems to have stopped talking to the till. The scanner is on and beeps when I scan anything but the till isn't registering it. Can you help me?

This is an engineer's nightmare!
Seen it so many times.
Before you ram cleared the till, you should have made a note of the com port settings
The first bit is easy - tell the com port in servi e mode that it has a scanner connected.
Then comes the fun
what baud rate is the scanner set to?
standard is 9600 but have seen 2400 and 19200 used too
then parity? odd? even? or none - different scanners use different settings here.
what about stop bits?

there are many parameters and they all have to match those in the scanner before they will talk to each other.
Unless you have a way of reprogramming or reading out the program in your scanner, I'm afraid it's just trial and error.