Nexus 5 stuck

Hi guys, i need some help with my Nexus 5. When i was using twrp i checked system by mistake while i wanted to wipe so now my phone has no OS installed. The problem is that if i try to go back to TWRP now my phone will go on the Google screen and won't open the custom recovery, so now i'm stuck either on the Google screen or in the menu where i can choose to start, go in recovery or to restart bootloader. What can i do to flash an OS and let the phone work again?
PLEASE HELP
Ok, now i can start the TWRP again, but i have still no OS, what can i do?