Are you by any chance on macOS and running an older version of MuseScore? There were some problems in the past with specific versions of macOS and MuseScore, but it should be fine with the current version, so be sure to update (3.2.3 is current). Also try Help / Revert to Factory Settings). If that doesn't fix it, please confirm you did those two things, and tell us what OS you are on and post a screenshot to see if that gives anyone any clues..
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Are you by any chance on macOS and running an older version of MuseScore? There were some problems in the past with specific versions of macOS and MuseScore, but it should be fine with the current version, so be sure to update (3.2.3 is current). Also try Help / Revert to Factory Settings). If that doesn't fix it, please confirm you did those two things, and tell us what OS you are on and post a screenshot to see if that gives anyone any clues..
There was such a bug in the Mac version of MuseScore 2.0.3 and got fixed in a Mac-only version 2.0.3.1 (see https://musescore.org/en/node/128076 and #126041: Black score view with MacOS Sierra), the latest 2.x version though is 2.3.2.