My USB modem is properly set and it used to work fine with Ubuntu 17.10. I tried it several times and it worked just fine. Now it suddenly decided not to work any more! It's working fine on other devices that have Windows but Ubuntu won't let it work. Anyone here knows how to address this issue?
Your question doesn't have enough information to go by or provide any help.
What's the make, model and type of your modem? What type of software you have used? What's your distribution? `lsb-release -a` What is the kernel version?
And many more such things. Also check
lsusb recognises your device. Perhaps replace cables and see if it works.