Q: why has mac stopped recognising scanner

Try removing it from Sys Prefs & adding it again.

Please help us help you. Mac model? Mac OS version? Scanner make and model???

Since you have provided zero information... In addition to the questions posed by Allan:

• Is the scanner turned on?
• Is the cable between the scanner and iMac connected at both ends?

check out > Mac Basics: Using a scanner - Apple Support