Without knowing more about your setup (computer specifications etc.) it's hard to know what might be wrong.
The one thing to try first is: launch Premiere Pro holding down the 'alt' key (on Windows) or the 'option' key on Mac. You will get a message asking to delete your preferences - click 'OK'.
Other things to try - open a new project and import your old project into the new project.
Others may have some suggestions - but please post more information about your (computer) setup and what you are editing (is it 4K, HD, what camera was used?). Which version of Premiere Pro are you using.