This happened to me today!
Do the following in order:

1.take battery out of back of phone
2. unplug all phone lines from phone, then unplug all phone lines from wall
3. unplug ac from phone, then from wall
Wait 30 seconds.
In this order:
1. replace battery
2. replace phone line(s) to phone, then to wall
3. replace ac to phone, then to wall.

These steps solved it for me! Good luck.