Hold the power button and volume-down until the device turns on. Then keep holding volume-down until the fastboot screen starts. Can you enter recovery? If so, clear cache. If not, get in fastboot mode, connect to USB, and run "fastboot erase cache". (You need your drivers and fastboot program set up on your computer.) If that doesn't allow booting to system and recovery, reinstall recovery using fastboot: fastboot flash recovery twrp.img. Then boot to TWRP (via fastboot, navigating the menus using the power and volume buttons). You can the nreinstall a ROM, and if needed, factory reset.