Hi
marcinmk76Thank you for your asking. Kindly provide the information below so that we could better assist you.
1. Does the issue appear after any system/apps download or upgrade?
2. Model Name: Settings>System>About phone
Refer to the link below to check the model name by model no. in the link
https://www.asus.com/support/FAQ/1014441/ 3. Could you still backup your data with cable to PC? Please backup your data and try factory reset by the following steps below:
a. Keep pressing "volume down key"
b. Press "power key" and stop press " power key" once you feel the vibration
c. Still keep pressing "volume down key" and stop press "volume down key" once you enter [Android Recovery] Screen
d. Choose "Wipe data/factory reset"
#Hardware Reset Guide
https://www.asus.com/support/FAQ/1006346/