I have a computer which displays non bootable device... can anyone tell me how to fix it?
You haven't even started to give us enough information to even start to try to help you.
Non-bootable device could be anything from a thumb drive to a hard drive.
Any device that is in the boot sequence. Modem, sound card, graphics card?
Not knowing what system you have...
Look in system devices and see if anything has a yellow exclamation mark by it. Click it and go into properties and update driver or uninstall it and scan for hardware changes or unhook, unplug the device and try to reboot.
You can also try rebooting after unplugging any peripherals that run on a usb port. If it boots, plug in one device at a time till the unboot warning comes back - that will be the device. Then look and see if it is compatible with your system. Check for updated software or drivers from the manufacturers.