Macbook Pro (Catalina) won't boot up ('do not enter' sign)

I opened my macbook and wanted to do some administration, but it was so slow I thought it might needed a restart.. This even didn't work so I tried the on/off button to shut it off.

When I turned it on again it gave me the 'do not enter' sign which quickly changed occasionally to the apple sign and back. I tried CMND + R, but this didn't work as well (white screen). What can I do? Is this a consequence of the Catalina update? My macbook worked perfectly before!!