Welcome to Android Central! If the Volume Up button is stuck in a pressed state, that might cause the phone to think you want to boot into Download Mode every time. Were you having problems with the Volume buttons before this happened?
Have you tried connecting it to a PC and issuing the fastboot reboot command? (You'd need ADB and fastboot installed.)
If that doesn't work either, you'd have to reflash the firmware - that would require LG's LGUP Tool to Flash KDZ and TOT Firmware and the KDZ or TOT file for your particular phone. (If fastboot reboot doesn't start the phone working, the firmware is too corrupt to start.)