the hardware requirements for 2020 are greater than for 2019. Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card. You might check your system to see if it meets requirements for 2020
It could also be that you need to update the drivers for your graphics card. My system was working fine for early versions of 2020 but after updating about a month ago, I got a message that my graphics card driver needed updating. There's a preference to turn this warning off and it's possible that yours is turned off.