Sometimes, when using a Raspberry pi, you may encounter display problems on your screen. In this case, there are 2 solutions :
I- Try to empty the browser cache
Just follow the steps explained here.
II- Flash the latest system image
If emptying the cache hasn't fixed the issue, then you have to flash the latest system image, just like you did when you first set up the Raspberry Pi
Download our system image here
Connect the micro SD card to your computer through the dedicated port or to a USB SD card reader
Download the Etcher software then install it on your computer
Flash the mawaqit.xz system image on the micro SD card with Etcher