Process System Not Responding message & Phones not working

Hi, My Nexus 5 suddenly was very low on power very quickly - while charging it, it looked like it wasn’t charging, but it was? then when I start the phone I usually get a message on screen “Process System Not Responding” “Stop - Wait - OK” whatever a choose makes no difference. the phone shows the background but no apps (usually) sometimes it starts then normally - then the message appears again.

Ive tried resetting the phone (long holding Power and Volume Down) but when I get message “No Command” and the little android guy on his side!

The phone is very well looked after and this happened very suddenly.

Can anyone help?

Thanks in advance