Deactivated service less than 90 days ago. Can’t reactivate modem

I deactivated my service less than 90 days ago.  Modem is still attached to coax cable.  I tried reactivating the modem last night and Xfinity can’t send signals.  Modem isn’t receiving anything.  Nothing has changed on my end, why wouldn’t they be able to reactivate the same modem that was just in service less than three months ago??  Now they want to send a technician?