When I clicked on the undo button it suddenly erased all of work on the one layer I was working on. I tried to click on the redo button it didn't work, I tried the undo button again , it didn't work. I tried to exit krita and open it again, it didnt work. The layer I was working on is now blank.
Please make sure you are using the latest version of Krita: 5.0.0 is pretty old by now. I'm also afraid that there is not enough information in this report for us to try and reproduce the issue, so we cannot investigate it.