You should be able to use fastboot to enter recovery mode and edit /system/build, depending on the Android device. prop to allow USB debugging after the system enters normal mode of operation.
- persist.service.adb.enable=1
- persist.service.debuggable=1
- persist.sys.usb.config=mtp,adb
On the other hand, Android devices can differ greatly and can have encrypted filesystems or be locked down. You will only be able to receive general, nonspecific suggestions that may or may not be helpful if you do not know the precise device and setup.
