My working solution is to hold the power button until the phone goes completely off. Wait a few seconds, turn it on ( usb connected) wait till the charging symbol appears and then power the phone back on.

Now i have a Nexus4 showing in mycomputer again.

Hold the power button until the phone turns off. Wait a few seconds. Then, turn it on, with the USB connected. Wait until the charging symbol appears. Then, turn on the phone.

Now, the Nexus 4 is showing in My computer again.