I guess you're referring to LG optimus L70.
You'll need to do a factory reset. Since you cannot go beyond boot logo, there is no point to instruct you to do backups. But check whether you can access your phone's files via a PC. LG pc suite may help deeper than windows explorer itself in this process.
Remove your SD card. SD card is unaffected by factory reset. Just do it for sure though.
The battery should be charged at least for 25%
1.Remove the battery. leave it for 2-3 seconds. and re insert
2.Hold volume up button and power button simultaneously.
3.The phone will boot. Once you see the LG logo, release ONLY the power key.then after 1 second, press the power key back again.(volume up key is pushed in always.)
4.then you will see "factory reset" thing. Follow the instructions.
** If this didn't help you, you will have to download the official ROM and flash the phone via LGFlashtool