Try starting up the phone in "Safe Mode" and see if it will let you access the phone and contacts in there. If so, then the problem may well be something on the phone. Virus, malware, bad app, etc. that will need to be removed to resolve it.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.